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<div class="section" id="logging-cookbook">
<span id="id1"></span><h1>Logging Cookbook<a class="headerlink" href="#logging-cookbook" title="Permalink to this headline">¶</a></h1>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field-odd field"><th class="field-name">Author:</th><td class="field-body">Vinay Sajip <vinay_sajip at red-dove dot com></td>
</tr>
</tbody>
</table>
<p>This page contains a number of recipes related to logging, which have been found
useful in the past.</p>
<div class="section" id="using-logging-in-multiple-modules">
<h2>Using logging in multiple modules<a class="headerlink" href="#using-logging-in-multiple-modules" title="Permalink to this headline">¶</a></h2>
<p>Multiple calls to <code class="docutils literal"><span class="pre">logging.getLogger('someLogger')</span></code> return a reference to the
same logger object. This is true not only within the same module, but also
across modules as long as it is in the same Python interpreter process. It is
true for references to the same object; additionally, application code can
define and configure a parent logger in one module and create (but not
configure) a child logger in a separate module, and all logger calls to the
child will pass up to the parent. Here is a main module:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">logging</span>
<span class="kn">import</span> <span class="nn">auxiliary_module</span>
<span class="c1"># create logger with 'spam_application'</span>
<span class="n">logger</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="s1">'spam_application'</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">setLevel</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">)</span>
<span class="c1"># create file handler which logs even debug messages</span>
<span class="n">fh</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">FileHandler</span><span class="p">(</span><span class="s1">'spam.log'</span><span class="p">)</span>
<span class="n">fh</span><span class="o">.</span><span class="n">setLevel</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">)</span>
<span class="c1"># create console handler with a higher log level</span>
<span class="n">ch</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">StreamHandler</span><span class="p">()</span>
<span class="n">ch</span><span class="o">.</span><span class="n">setLevel</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">ERROR</span><span class="p">)</span>
<span class="c1"># create formatter and add it to the handlers</span>
<span class="n">formatter</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">Formatter</span><span class="p">(</span><span class="s1">'</span><span class="si">%(asctime)s</span><span class="s1"> - </span><span class="si">%(name)s</span><span class="s1"> - </span><span class="si">%(levelname)s</span><span class="s1"> - </span><span class="si">%(message)s</span><span class="s1">'</span><span class="p">)</span>
<span class="n">fh</span><span class="o">.</span><span class="n">setFormatter</span><span class="p">(</span><span class="n">formatter</span><span class="p">)</span>
<span class="n">ch</span><span class="o">.</span><span class="n">setFormatter</span><span class="p">(</span><span class="n">formatter</span><span class="p">)</span>
<span class="c1"># add the handlers to the logger</span>
<span class="n">logger</span><span class="o">.</span><span class="n">addHandler</span><span class="p">(</span><span class="n">fh</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">addHandler</span><span class="p">(</span><span class="n">ch</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'creating an instance of auxiliary_module.Auxiliary'</span><span class="p">)</span>
<span class="n">a</span> <span class="o">=</span> <span class="n">auxiliary_module</span><span class="o">.</span><span class="n">Auxiliary</span><span class="p">()</span>
<span class="n">logger</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'created an instance of auxiliary_module.Auxiliary'</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'calling auxiliary_module.Auxiliary.do_something'</span><span class="p">)</span>
<span class="n">a</span><span class="o">.</span><span class="n">do_something</span><span class="p">()</span>
<span class="n">logger</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'finished auxiliary_module.Auxiliary.do_something'</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'calling auxiliary_module.some_function()'</span><span class="p">)</span>
<span class="n">auxiliary_module</span><span class="o">.</span><span class="n">some_function</span><span class="p">()</span>
<span class="n">logger</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'done with auxiliary_module.some_function()'</span><span class="p">)</span>
</pre></div>
</div>
<p>Here is the auxiliary module:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">logging</span>
<span class="c1"># create logger</span>
<span class="n">module_logger</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="s1">'spam_application.auxiliary'</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">Auxiliary</span><span class="p">:</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">logger</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="s1">'spam_application.auxiliary.Auxiliary'</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">logger</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'creating an instance of Auxiliary'</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">do_something</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">logger</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'doing something'</span><span class="p">)</span>
<span class="n">a</span> <span class="o">=</span> <span class="mi">1</span> <span class="o">+</span> <span class="mi">1</span>
<span class="bp">self</span><span class="o">.</span><span class="n">logger</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'done doing something'</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">some_function</span><span class="p">():</span>
<span class="n">module_logger</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'received a call to "some_function"'</span><span class="p">)</span>
</pre></div>
</div>
<p>The output looks like this:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="mi">2005</span><span class="o">-</span><span class="mi">03</span><span class="o">-</span><span class="mi">23</span> <span class="mi">23</span><span class="p">:</span><span class="mi">47</span><span class="p">:</span><span class="mi">11</span><span class="p">,</span><span class="mi">663</span> <span class="o">-</span> <span class="n">spam_application</span> <span class="o">-</span> <span class="n">INFO</span> <span class="o">-</span>
<span class="n">creating</span> <span class="n">an</span> <span class="n">instance</span> <span class="n">of</span> <span class="n">auxiliary_module</span><span class="o">.</span><span class="n">Auxiliary</span>
<span class="mi">2005</span><span class="o">-</span><span class="mi">03</span><span class="o">-</span><span class="mi">23</span> <span class="mi">23</span><span class="p">:</span><span class="mi">47</span><span class="p">:</span><span class="mi">11</span><span class="p">,</span><span class="mi">665</span> <span class="o">-</span> <span class="n">spam_application</span><span class="o">.</span><span class="n">auxiliary</span><span class="o">.</span><span class="n">Auxiliary</span> <span class="o">-</span> <span class="n">INFO</span> <span class="o">-</span>
<span class="n">creating</span> <span class="n">an</span> <span class="n">instance</span> <span class="n">of</span> <span class="n">Auxiliary</span>
<span class="mi">2005</span><span class="o">-</span><span class="mi">03</span><span class="o">-</span><span class="mi">23</span> <span class="mi">23</span><span class="p">:</span><span class="mi">47</span><span class="p">:</span><span class="mi">11</span><span class="p">,</span><span class="mi">665</span> <span class="o">-</span> <span class="n">spam_application</span> <span class="o">-</span> <span class="n">INFO</span> <span class="o">-</span>
<span class="n">created</span> <span class="n">an</span> <span class="n">instance</span> <span class="n">of</span> <span class="n">auxiliary_module</span><span class="o">.</span><span class="n">Auxiliary</span>
<span class="mi">2005</span><span class="o">-</span><span class="mi">03</span><span class="o">-</span><span class="mi">23</span> <span class="mi">23</span><span class="p">:</span><span class="mi">47</span><span class="p">:</span><span class="mi">11</span><span class="p">,</span><span class="mi">668</span> <span class="o">-</span> <span class="n">spam_application</span> <span class="o">-</span> <span class="n">INFO</span> <span class="o">-</span>
<span class="n">calling</span> <span class="n">auxiliary_module</span><span class="o">.</span><span class="n">Auxiliary</span><span class="o">.</span><span class="n">do_something</span>
<span class="mi">2005</span><span class="o">-</span><span class="mi">03</span><span class="o">-</span><span class="mi">23</span> <span class="mi">23</span><span class="p">:</span><span class="mi">47</span><span class="p">:</span><span class="mi">11</span><span class="p">,</span><span class="mi">668</span> <span class="o">-</span> <span class="n">spam_application</span><span class="o">.</span><span class="n">auxiliary</span><span class="o">.</span><span class="n">Auxiliary</span> <span class="o">-</span> <span class="n">INFO</span> <span class="o">-</span>
<span class="n">doing</span> <span class="n">something</span>
<span class="mi">2005</span><span class="o">-</span><span class="mi">03</span><span class="o">-</span><span class="mi">23</span> <span class="mi">23</span><span class="p">:</span><span class="mi">47</span><span class="p">:</span><span class="mi">11</span><span class="p">,</span><span class="mi">669</span> <span class="o">-</span> <span class="n">spam_application</span><span class="o">.</span><span class="n">auxiliary</span><span class="o">.</span><span class="n">Auxiliary</span> <span class="o">-</span> <span class="n">INFO</span> <span class="o">-</span>
<span class="n">done</span> <span class="n">doing</span> <span class="n">something</span>
<span class="mi">2005</span><span class="o">-</span><span class="mi">03</span><span class="o">-</span><span class="mi">23</span> <span class="mi">23</span><span class="p">:</span><span class="mi">47</span><span class="p">:</span><span class="mi">11</span><span class="p">,</span><span class="mi">670</span> <span class="o">-</span> <span class="n">spam_application</span> <span class="o">-</span> <span class="n">INFO</span> <span class="o">-</span>
<span class="n">finished</span> <span class="n">auxiliary_module</span><span class="o">.</span><span class="n">Auxiliary</span><span class="o">.</span><span class="n">do_something</span>
<span class="mi">2005</span><span class="o">-</span><span class="mi">03</span><span class="o">-</span><span class="mi">23</span> <span class="mi">23</span><span class="p">:</span><span class="mi">47</span><span class="p">:</span><span class="mi">11</span><span class="p">,</span><span class="mi">671</span> <span class="o">-</span> <span class="n">spam_application</span> <span class="o">-</span> <span class="n">INFO</span> <span class="o">-</span>
<span class="n">calling</span> <span class="n">auxiliary_module</span><span class="o">.</span><span class="n">some_function</span><span class="p">()</span>
<span class="mi">2005</span><span class="o">-</span><span class="mi">03</span><span class="o">-</span><span class="mi">23</span> <span class="mi">23</span><span class="p">:</span><span class="mi">47</span><span class="p">:</span><span class="mi">11</span><span class="p">,</span><span class="mi">672</span> <span class="o">-</span> <span class="n">spam_application</span><span class="o">.</span><span class="n">auxiliary</span> <span class="o">-</span> <span class="n">INFO</span> <span class="o">-</span>
<span class="n">received</span> <span class="n">a</span> <span class="n">call</span> <span class="n">to</span> <span class="s1">'some_function'</span>
<span class="mi">2005</span><span class="o">-</span><span class="mi">03</span><span class="o">-</span><span class="mi">23</span> <span class="mi">23</span><span class="p">:</span><span class="mi">47</span><span class="p">:</span><span class="mi">11</span><span class="p">,</span><span class="mi">673</span> <span class="o">-</span> <span class="n">spam_application</span> <span class="o">-</span> <span class="n">INFO</span> <span class="o">-</span>
<span class="n">done</span> <span class="k">with</span> <span class="n">auxiliary_module</span><span class="o">.</span><span class="n">some_function</span><span class="p">()</span>
</pre></div>
</div>
</div>
<div class="section" id="logging-from-multiple-threads">
<h2>Logging from multiple threads<a class="headerlink" href="#logging-from-multiple-threads" title="Permalink to this headline">¶</a></h2>
<p>Logging from multiple threads requires no special effort. The following example
shows logging from the main (initial) thread and another thread:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">logging</span>
<span class="kn">import</span> <span class="nn">threading</span>
<span class="kn">import</span> <span class="nn">time</span>
<span class="k">def</span> <span class="nf">worker</span><span class="p">(</span><span class="n">arg</span><span class="p">):</span>
<span class="k">while</span> <span class="ow">not</span> <span class="n">arg</span><span class="p">[</span><span class="s1">'stop'</span><span class="p">]:</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s1">'Hi from myfunc'</span><span class="p">)</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mf">0.5</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">main</span><span class="p">():</span>
<span class="n">logging</span><span class="o">.</span><span class="n">basicConfig</span><span class="p">(</span><span class="n">level</span><span class="o">=</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">,</span> <span class="nb">format</span><span class="o">=</span><span class="s1">'</span><span class="si">%(relativeCreated)6d</span><span class="s1"> </span><span class="si">%(threadName)s</span><span class="s1"> </span><span class="si">%(message)s</span><span class="s1">'</span><span class="p">)</span>
<span class="n">info</span> <span class="o">=</span> <span class="p">{</span><span class="s1">'stop'</span><span class="p">:</span> <span class="kc">False</span><span class="p">}</span>
<span class="n">thread</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">worker</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">info</span><span class="p">,))</span>
<span class="n">thread</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="k">while</span> <span class="kc">True</span><span class="p">:</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s1">'Hello from main'</span><span class="p">)</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mf">0.75</span><span class="p">)</span>
<span class="k">except</span> <span class="ne">KeyboardInterrupt</span><span class="p">:</span>
<span class="n">info</span><span class="p">[</span><span class="s1">'stop'</span><span class="p">]</span> <span class="o">=</span> <span class="kc">True</span>
<span class="k">break</span>
<span class="n">thread</span><span class="o">.</span><span class="n">join</span><span class="p">()</span>
<span class="k">if</span> <span class="n">__name__</span> <span class="o">==</span> <span class="s1">'__main__'</span><span class="p">:</span>
<span class="n">main</span><span class="p">()</span>
</pre></div>
</div>
<p>When run, the script should print something like the following:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span> <span class="mi">0</span> <span class="n">Thread</span><span class="o">-</span><span class="mi">1</span> <span class="n">Hi</span> <span class="kn">from</span> <span class="nn">myfunc</span>
<span class="mi">3</span> <span class="n">MainThread</span> <span class="n">Hello</span> <span class="kn">from</span> <span class="nn">main</span>
<span class="mi">505</span> <span class="n">Thread</span><span class="o">-</span><span class="mi">1</span> <span class="n">Hi</span> <span class="kn">from</span> <span class="nn">myfunc</span>
<span class="mi">755</span> <span class="n">MainThread</span> <span class="n">Hello</span> <span class="kn">from</span> <span class="nn">main</span>
<span class="mi">1007</span> <span class="n">Thread</span><span class="o">-</span><span class="mi">1</span> <span class="n">Hi</span> <span class="kn">from</span> <span class="nn">myfunc</span>
<span class="mi">1507</span> <span class="n">MainThread</span> <span class="n">Hello</span> <span class="kn">from</span> <span class="nn">main</span>
<span class="mi">1508</span> <span class="n">Thread</span><span class="o">-</span><span class="mi">1</span> <span class="n">Hi</span> <span class="kn">from</span> <span class="nn">myfunc</span>
<span class="mi">2010</span> <span class="n">Thread</span><span class="o">-</span><span class="mi">1</span> <span class="n">Hi</span> <span class="kn">from</span> <span class="nn">myfunc</span>
<span class="mi">2258</span> <span class="n">MainThread</span> <span class="n">Hello</span> <span class="kn">from</span> <span class="nn">main</span>
<span class="mi">2512</span> <span class="n">Thread</span><span class="o">-</span><span class="mi">1</span> <span class="n">Hi</span> <span class="kn">from</span> <span class="nn">myfunc</span>
<span class="mi">3009</span> <span class="n">MainThread</span> <span class="n">Hello</span> <span class="kn">from</span> <span class="nn">main</span>
<span class="mi">3013</span> <span class="n">Thread</span><span class="o">-</span><span class="mi">1</span> <span class="n">Hi</span> <span class="kn">from</span> <span class="nn">myfunc</span>
<span class="mi">3515</span> <span class="n">Thread</span><span class="o">-</span><span class="mi">1</span> <span class="n">Hi</span> <span class="kn">from</span> <span class="nn">myfunc</span>
<span class="mi">3761</span> <span class="n">MainThread</span> <span class="n">Hello</span> <span class="kn">from</span> <span class="nn">main</span>
<span class="mi">4017</span> <span class="n">Thread</span><span class="o">-</span><span class="mi">1</span> <span class="n">Hi</span> <span class="kn">from</span> <span class="nn">myfunc</span>
<span class="mi">4513</span> <span class="n">MainThread</span> <span class="n">Hello</span> <span class="kn">from</span> <span class="nn">main</span>
<span class="mi">4518</span> <span class="n">Thread</span><span class="o">-</span><span class="mi">1</span> <span class="n">Hi</span> <span class="kn">from</span> <span class="nn">myfunc</span>
</pre></div>
</div>
<p>This shows the logging output interspersed as one might expect. This approach
works for more threads than shown here, of course.</p>
</div>
<div class="section" id="multiple-handlers-and-formatters">
<h2>Multiple handlers and formatters<a class="headerlink" href="#multiple-handlers-and-formatters" title="Permalink to this headline">¶</a></h2>
<p>Loggers are plain Python objects. The <a class="reference internal" href="../library/logging.html#logging.Logger.addHandler" title="logging.Logger.addHandler"><code class="xref py py-meth docutils literal"><span class="pre">addHandler()</span></code></a> method has no
minimum or maximum quota for the number of handlers you may add. Sometimes it
will be beneficial for an application to log all messages of all severities to a
text file while simultaneously logging errors or above to the console. To set
this up, simply configure the appropriate handlers. The logging calls in the
application code will remain unchanged. Here is a slight modification to the
previous simple module-based configuration example:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">logging</span>
<span class="n">logger</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="s1">'simple_example'</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">setLevel</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">)</span>
<span class="c1"># create file handler which logs even debug messages</span>
<span class="n">fh</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">FileHandler</span><span class="p">(</span><span class="s1">'spam.log'</span><span class="p">)</span>
<span class="n">fh</span><span class="o">.</span><span class="n">setLevel</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">)</span>
<span class="c1"># create console handler with a higher log level</span>
<span class="n">ch</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">StreamHandler</span><span class="p">()</span>
<span class="n">ch</span><span class="o">.</span><span class="n">setLevel</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">ERROR</span><span class="p">)</span>
<span class="c1"># create formatter and add it to the handlers</span>
<span class="n">formatter</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">Formatter</span><span class="p">(</span><span class="s1">'</span><span class="si">%(asctime)s</span><span class="s1"> - </span><span class="si">%(name)s</span><span class="s1"> - </span><span class="si">%(levelname)s</span><span class="s1"> - </span><span class="si">%(message)s</span><span class="s1">'</span><span class="p">)</span>
<span class="n">ch</span><span class="o">.</span><span class="n">setFormatter</span><span class="p">(</span><span class="n">formatter</span><span class="p">)</span>
<span class="n">fh</span><span class="o">.</span><span class="n">setFormatter</span><span class="p">(</span><span class="n">formatter</span><span class="p">)</span>
<span class="c1"># add the handlers to logger</span>
<span class="n">logger</span><span class="o">.</span><span class="n">addHandler</span><span class="p">(</span><span class="n">ch</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">addHandler</span><span class="p">(</span><span class="n">fh</span><span class="p">)</span>
<span class="c1"># 'application' code</span>
<span class="n">logger</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s1">'debug message'</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'info message'</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">warn</span><span class="p">(</span><span class="s1">'warn message'</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">error</span><span class="p">(</span><span class="s1">'error message'</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">critical</span><span class="p">(</span><span class="s1">'critical message'</span><span class="p">)</span>
</pre></div>
</div>
<p>Notice that the ‘application’ code does not care about multiple handlers. All
that changed was the addition and configuration of a new handler named <em>fh</em>.</p>
<p>The ability to create new handlers with higher- or lower-severity filters can be
very helpful when writing and testing an application. Instead of using many
<code class="docutils literal"><span class="pre">print</span></code> statements for debugging, use <code class="docutils literal"><span class="pre">logger.debug</span></code>: Unlike the print
statements, which you will have to delete or comment out later, the logger.debug
statements can remain intact in the source code and remain dormant until you
need them again. At that time, the only change that needs to happen is to
modify the severity level of the logger and/or handler to debug.</p>
</div>
<div class="section" id="logging-to-multiple-destinations">
<span id="multiple-destinations"></span><h2>Logging to multiple destinations<a class="headerlink" href="#logging-to-multiple-destinations" title="Permalink to this headline">¶</a></h2>
<p>Let’s say you want to log to console and file with different message formats and
in differing circumstances. Say you want to log messages with levels of DEBUG
and higher to file, and those messages at level INFO and higher to the console.
Let’s also assume that the file should contain timestamps, but the console
messages should not. Here’s how you can achieve this:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">logging</span>
<span class="c1"># set up logging to file - see previous section for more details</span>
<span class="n">logging</span><span class="o">.</span><span class="n">basicConfig</span><span class="p">(</span><span class="n">level</span><span class="o">=</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">,</span>
<span class="nb">format</span><span class="o">=</span><span class="s1">'</span><span class="si">%(asctime)s</span><span class="s1"> </span><span class="si">%(name)-12s</span><span class="s1"> </span><span class="si">%(levelname)-8s</span><span class="s1"> </span><span class="si">%(message)s</span><span class="s1">'</span><span class="p">,</span>
<span class="n">datefmt</span><span class="o">=</span><span class="s1">'%m-</span><span class="si">%d</span><span class="s1"> %H:%M'</span><span class="p">,</span>
<span class="n">filename</span><span class="o">=</span><span class="s1">'/temp/myapp.log'</span><span class="p">,</span>
<span class="n">filemode</span><span class="o">=</span><span class="s1">'w'</span><span class="p">)</span>
<span class="c1"># define a Handler which writes INFO messages or higher to the sys.stderr</span>
<span class="n">console</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">StreamHandler</span><span class="p">()</span>
<span class="n">console</span><span class="o">.</span><span class="n">setLevel</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">INFO</span><span class="p">)</span>
<span class="c1"># set a format which is simpler for console use</span>
<span class="n">formatter</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">Formatter</span><span class="p">(</span><span class="s1">'</span><span class="si">%(name)-12s</span><span class="s1">: </span><span class="si">%(levelname)-8s</span><span class="s1"> </span><span class="si">%(message)s</span><span class="s1">'</span><span class="p">)</span>
<span class="c1"># tell the handler to use this format</span>
<span class="n">console</span><span class="o">.</span><span class="n">setFormatter</span><span class="p">(</span><span class="n">formatter</span><span class="p">)</span>
<span class="c1"># add the handler to the root logger</span>
<span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="s1">''</span><span class="p">)</span><span class="o">.</span><span class="n">addHandler</span><span class="p">(</span><span class="n">console</span><span class="p">)</span>
<span class="c1"># Now, we can log to the root logger, or any other logger. First the root...</span>
<span class="n">logging</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'Jackdaws love my big sphinx of quartz.'</span><span class="p">)</span>
<span class="c1"># Now, define a couple of other loggers which might represent areas in your</span>
<span class="c1"># application:</span>
<span class="n">logger1</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="s1">'myapp.area1'</span><span class="p">)</span>
<span class="n">logger2</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="s1">'myapp.area2'</span><span class="p">)</span>
<span class="n">logger1</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s1">'Quick zephyrs blow, vexing daft Jim.'</span><span class="p">)</span>
<span class="n">logger1</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'How quickly daft jumping zebras vex.'</span><span class="p">)</span>
<span class="n">logger2</span><span class="o">.</span><span class="n">warning</span><span class="p">(</span><span class="s1">'Jail zesty vixen who grabbed pay from quack.'</span><span class="p">)</span>
<span class="n">logger2</span><span class="o">.</span><span class="n">error</span><span class="p">(</span><span class="s1">'The five boxing wizards jump quickly.'</span><span class="p">)</span>
</pre></div>
</div>
<p>When you run this, on the console you will see</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">root</span> <span class="p">:</span> <span class="n">INFO</span> <span class="n">Jackdaws</span> <span class="n">love</span> <span class="n">my</span> <span class="n">big</span> <span class="n">sphinx</span> <span class="n">of</span> <span class="n">quartz</span><span class="o">.</span>
<span class="n">myapp</span><span class="o">.</span><span class="n">area1</span> <span class="p">:</span> <span class="n">INFO</span> <span class="n">How</span> <span class="n">quickly</span> <span class="n">daft</span> <span class="n">jumping</span> <span class="n">zebras</span> <span class="n">vex</span><span class="o">.</span>
<span class="n">myapp</span><span class="o">.</span><span class="n">area2</span> <span class="p">:</span> <span class="n">WARNING</span> <span class="n">Jail</span> <span class="n">zesty</span> <span class="n">vixen</span> <span class="n">who</span> <span class="n">grabbed</span> <span class="n">pay</span> <span class="kn">from</span> <span class="nn">quack.</span>
<span class="n">myapp</span><span class="o">.</span><span class="n">area2</span> <span class="p">:</span> <span class="n">ERROR</span> <span class="n">The</span> <span class="n">five</span> <span class="n">boxing</span> <span class="n">wizards</span> <span class="n">jump</span> <span class="n">quickly</span><span class="o">.</span>
</pre></div>
</div>
<p>and in the file you will see something like</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="mi">10</span><span class="o">-</span><span class="mi">22</span> <span class="mi">22</span><span class="p">:</span><span class="mi">19</span> <span class="n">root</span> <span class="n">INFO</span> <span class="n">Jackdaws</span> <span class="n">love</span> <span class="n">my</span> <span class="n">big</span> <span class="n">sphinx</span> <span class="n">of</span> <span class="n">quartz</span><span class="o">.</span>
<span class="mi">10</span><span class="o">-</span><span class="mi">22</span> <span class="mi">22</span><span class="p">:</span><span class="mi">19</span> <span class="n">myapp</span><span class="o">.</span><span class="n">area1</span> <span class="n">DEBUG</span> <span class="n">Quick</span> <span class="n">zephyrs</span> <span class="n">blow</span><span class="p">,</span> <span class="n">vexing</span> <span class="n">daft</span> <span class="n">Jim</span><span class="o">.</span>
<span class="mi">10</span><span class="o">-</span><span class="mi">22</span> <span class="mi">22</span><span class="p">:</span><span class="mi">19</span> <span class="n">myapp</span><span class="o">.</span><span class="n">area1</span> <span class="n">INFO</span> <span class="n">How</span> <span class="n">quickly</span> <span class="n">daft</span> <span class="n">jumping</span> <span class="n">zebras</span> <span class="n">vex</span><span class="o">.</span>
<span class="mi">10</span><span class="o">-</span><span class="mi">22</span> <span class="mi">22</span><span class="p">:</span><span class="mi">19</span> <span class="n">myapp</span><span class="o">.</span><span class="n">area2</span> <span class="n">WARNING</span> <span class="n">Jail</span> <span class="n">zesty</span> <span class="n">vixen</span> <span class="n">who</span> <span class="n">grabbed</span> <span class="n">pay</span> <span class="kn">from</span> <span class="nn">quack.</span>
<span class="mi">10</span><span class="o">-</span><span class="mi">22</span> <span class="mi">22</span><span class="p">:</span><span class="mi">19</span> <span class="n">myapp</span><span class="o">.</span><span class="n">area2</span> <span class="n">ERROR</span> <span class="n">The</span> <span class="n">five</span> <span class="n">boxing</span> <span class="n">wizards</span> <span class="n">jump</span> <span class="n">quickly</span><span class="o">.</span>
</pre></div>
</div>
<p>As you can see, the DEBUG message only shows up in the file. The other messages
are sent to both destinations.</p>
<p>This example uses console and file handlers, but you can use any number and
combination of handlers you choose.</p>
</div>
<div class="section" id="configuration-server-example">
<h2>Configuration server example<a class="headerlink" href="#configuration-server-example" title="Permalink to this headline">¶</a></h2>
<p>Here is an example of a module using the logging configuration server:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">logging</span>
<span class="kn">import</span> <span class="nn">logging.config</span>
<span class="kn">import</span> <span class="nn">time</span>
<span class="kn">import</span> <span class="nn">os</span>
<span class="c1"># read initial config file</span>
<span class="n">logging</span><span class="o">.</span><span class="n">config</span><span class="o">.</span><span class="n">fileConfig</span><span class="p">(</span><span class="s1">'logging.conf'</span><span class="p">)</span>
<span class="c1"># create and start listener on port 9999</span>
<span class="n">t</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">config</span><span class="o">.</span><span class="n">listen</span><span class="p">(</span><span class="mi">9999</span><span class="p">)</span>
<span class="n">t</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="n">logger</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="s1">'simpleExample'</span><span class="p">)</span>
<span class="k">try</span><span class="p">:</span>
<span class="c1"># loop through logging calls to see the difference</span>
<span class="c1"># new configurations make, until Ctrl+C is pressed</span>
<span class="k">while</span> <span class="kc">True</span><span class="p">:</span>
<span class="n">logger</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s1">'debug message'</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'info message'</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">warn</span><span class="p">(</span><span class="s1">'warn message'</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">error</span><span class="p">(</span><span class="s1">'error message'</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">critical</span><span class="p">(</span><span class="s1">'critical message'</span><span class="p">)</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span>
<span class="k">except</span> <span class="ne">KeyboardInterrupt</span><span class="p">:</span>
<span class="c1"># cleanup</span>
<span class="n">logging</span><span class="o">.</span><span class="n">config</span><span class="o">.</span><span class="n">stopListening</span><span class="p">()</span>
<span class="n">t</span><span class="o">.</span><span class="n">join</span><span class="p">()</span>
</pre></div>
</div>
<p>And here is a script that takes a filename and sends that file to the server,
properly preceded with the binary-encoded length, as the new logging
configuration:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="ch">#!/usr/bin/env python</span>
<span class="kn">import</span> <span class="nn">socket</span><span class="o">,</span> <span class="nn">sys</span><span class="o">,</span> <span class="nn">struct</span>
<span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="n">sys</span><span class="o">.</span><span class="n">argv</span><span class="p">[</span><span class="mi">1</span><span class="p">],</span> <span class="s1">'rb'</span><span class="p">)</span> <span class="k">as</span> <span class="n">f</span><span class="p">:</span>
<span class="n">data_to_send</span> <span class="o">=</span> <span class="n">f</span><span class="o">.</span><span class="n">read</span><span class="p">()</span>
<span class="n">HOST</span> <span class="o">=</span> <span class="s1">'localhost'</span>
<span class="n">PORT</span> <span class="o">=</span> <span class="mi">9999</span>
<span class="n">s</span> <span class="o">=</span> <span class="n">socket</span><span class="o">.</span><span class="n">socket</span><span class="p">(</span><span class="n">socket</span><span class="o">.</span><span class="n">AF_INET</span><span class="p">,</span> <span class="n">socket</span><span class="o">.</span><span class="n">SOCK_STREAM</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="s1">'connecting...'</span><span class="p">)</span>
<span class="n">s</span><span class="o">.</span><span class="n">connect</span><span class="p">((</span><span class="n">HOST</span><span class="p">,</span> <span class="n">PORT</span><span class="p">))</span>
<span class="nb">print</span><span class="p">(</span><span class="s1">'sending config...'</span><span class="p">)</span>
<span class="n">s</span><span class="o">.</span><span class="n">send</span><span class="p">(</span><span class="n">struct</span><span class="o">.</span><span class="n">pack</span><span class="p">(</span><span class="s1">'>L'</span><span class="p">,</span> <span class="nb">len</span><span class="p">(</span><span class="n">data_to_send</span><span class="p">)))</span>
<span class="n">s</span><span class="o">.</span><span class="n">send</span><span class="p">(</span><span class="n">data_to_send</span><span class="p">)</span>
<span class="n">s</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
<span class="nb">print</span><span class="p">(</span><span class="s1">'complete'</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="section" id="dealing-with-handlers-that-block">
<h2>Dealing with handlers that block<a class="headerlink" href="#dealing-with-handlers-that-block" title="Permalink to this headline">¶</a></h2>
<p>Sometimes you have to get your logging handlers to do their work without
blocking the thread you’re logging from. This is common in Web applications,
though of course it also occurs in other scenarios.</p>
<p>A common culprit which demonstrates sluggish behaviour is the
<a class="reference internal" href="../library/logging.handlers.html#logging.handlers.SMTPHandler" title="logging.handlers.SMTPHandler"><code class="xref py py-class docutils literal"><span class="pre">SMTPHandler</span></code></a>: sending emails can take a long time, for a
number of reasons outside the developer’s control (for example, a poorly
performing mail or network infrastructure). But almost any network-based
handler can block: Even a <a class="reference internal" href="../library/logging.handlers.html#logging.handlers.SocketHandler" title="logging.handlers.SocketHandler"><code class="xref py py-class docutils literal"><span class="pre">SocketHandler</span></code></a> operation may do a
DNS query under the hood which is too slow (and this query can be deep in the
socket library code, below the Python layer, and outside your control).</p>
<p>One solution is to use a two-part approach. For the first part, attach only a
<a class="reference internal" href="../library/logging.handlers.html#logging.handlers.QueueHandler" title="logging.handlers.QueueHandler"><code class="xref py py-class docutils literal"><span class="pre">QueueHandler</span></code></a> to those loggers which are accessed from
performance-critical threads. They simply write to their queue, which can be
sized to a large enough capacity or initialized with no upper bound to their
size. The write to the queue will typically be accepted quickly, though you
will probably need to catch the <a class="reference internal" href="../library/queue.html#queue.Full" title="queue.Full"><code class="xref py py-exc docutils literal"><span class="pre">queue.Full</span></code></a> exception as a precaution
in your code. If you are a library developer who has performance-critical
threads in their code, be sure to document this (together with a suggestion to
attach only <code class="docutils literal"><span class="pre">QueueHandlers</span></code> to your loggers) for the benefit of other
developers who will use your code.</p>
<p>The second part of the solution is <a class="reference internal" href="../library/logging.handlers.html#logging.handlers.QueueListener" title="logging.handlers.QueueListener"><code class="xref py py-class docutils literal"><span class="pre">QueueListener</span></code></a>, which has been
designed as the counterpart to <a class="reference internal" href="../library/logging.handlers.html#logging.handlers.QueueHandler" title="logging.handlers.QueueHandler"><code class="xref py py-class docutils literal"><span class="pre">QueueHandler</span></code></a>. A
<a class="reference internal" href="../library/logging.handlers.html#logging.handlers.QueueListener" title="logging.handlers.QueueListener"><code class="xref py py-class docutils literal"><span class="pre">QueueListener</span></code></a> is very simple: it’s passed a queue and some handlers,
and it fires up an internal thread which listens to its queue for LogRecords
sent from <code class="docutils literal"><span class="pre">QueueHandlers</span></code> (or any other source of <code class="docutils literal"><span class="pre">LogRecords</span></code>, for that
matter). The <code class="docutils literal"><span class="pre">LogRecords</span></code> are removed from the queue and passed to the
handlers for processing.</p>
<p>The advantage of having a separate <a class="reference internal" href="../library/logging.handlers.html#logging.handlers.QueueListener" title="logging.handlers.QueueListener"><code class="xref py py-class docutils literal"><span class="pre">QueueListener</span></code></a> class is that you
can use the same instance to service multiple <code class="docutils literal"><span class="pre">QueueHandlers</span></code>. This is more
resource-friendly than, say, having threaded versions of the existing handler
classes, which would eat up one thread per handler for no particular benefit.</p>
<p>An example of using these two classes follows (imports omitted):</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">que</span> <span class="o">=</span> <span class="n">queue</span><span class="o">.</span><span class="n">Queue</span><span class="p">(</span><span class="o">-</span><span class="mi">1</span><span class="p">)</span> <span class="c1"># no limit on size</span>
<span class="n">queue_handler</span> <span class="o">=</span> <span class="n">QueueHandler</span><span class="p">(</span><span class="n">que</span><span class="p">)</span>
<span class="n">handler</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">StreamHandler</span><span class="p">()</span>
<span class="n">listener</span> <span class="o">=</span> <span class="n">QueueListener</span><span class="p">(</span><span class="n">que</span><span class="p">,</span> <span class="n">handler</span><span class="p">)</span>
<span class="n">root</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">()</span>
<span class="n">root</span><span class="o">.</span><span class="n">addHandler</span><span class="p">(</span><span class="n">queue_handler</span><span class="p">)</span>
<span class="n">formatter</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">Formatter</span><span class="p">(</span><span class="s1">'</span><span class="si">%(threadName)s</span><span class="s1">: </span><span class="si">%(message)s</span><span class="s1">'</span><span class="p">)</span>
<span class="n">handler</span><span class="o">.</span><span class="n">setFormatter</span><span class="p">(</span><span class="n">formatter</span><span class="p">)</span>
<span class="n">listener</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="c1"># The log output will display the thread which generated</span>
<span class="c1"># the event (the main thread) rather than the internal</span>
<span class="c1"># thread which monitors the internal queue. This is what</span>
<span class="c1"># you want to happen.</span>
<span class="n">root</span><span class="o">.</span><span class="n">warning</span><span class="p">(</span><span class="s1">'Look out!'</span><span class="p">)</span>
<span class="n">listener</span><span class="o">.</span><span class="n">stop</span><span class="p">()</span>
</pre></div>
</div>
<p>which, when run, will produce:</p>
<div class="highlight-none"><div class="highlight"><pre><span></span>MainThread: Look out!
</pre></div>
</div>
<div class="versionchanged">
<p><span class="versionmodified">Changed in version 3.5: </span>Prior to Python 3.5, the <a class="reference internal" href="../library/logging.handlers.html#logging.handlers.QueueListener" title="logging.handlers.QueueListener"><code class="xref py py-class docutils literal"><span class="pre">QueueListener</span></code></a> always passed every message
received from the queue to every handler it was initialized with. (This was
because it was assumed that level filtering was all done on the other side,
where the queue is filled.) From 3.5 onwards, this behaviour can be changed
by passing a keyword argument <code class="docutils literal"><span class="pre">respect_handler_level=True</span></code> to the
listener’s constructor. When this is done, the listener compares the level
of each message with the handler’s level, and only passes a message to a
handler if it’s appropriate to do so.</p>
</div>
</div>
<div class="section" id="sending-and-receiving-logging-events-across-a-network">
<span id="network-logging"></span><h2>Sending and receiving logging events across a network<a class="headerlink" href="#sending-and-receiving-logging-events-across-a-network" title="Permalink to this headline">¶</a></h2>
<p>Let’s say you want to send logging events across a network, and handle them at
the receiving end. A simple way of doing this is attaching a
<a class="reference internal" href="../library/logging.handlers.html#logging.handlers.SocketHandler" title="logging.handlers.SocketHandler"><code class="xref py py-class docutils literal"><span class="pre">SocketHandler</span></code></a> instance to the root logger at the sending end:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">logging</span><span class="o">,</span> <span class="nn">logging.handlers</span>
<span class="n">rootLogger</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="s1">''</span><span class="p">)</span>
<span class="n">rootLogger</span><span class="o">.</span><span class="n">setLevel</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">)</span>
<span class="n">socketHandler</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">handlers</span><span class="o">.</span><span class="n">SocketHandler</span><span class="p">(</span><span class="s1">'localhost'</span><span class="p">,</span>
<span class="n">logging</span><span class="o">.</span><span class="n">handlers</span><span class="o">.</span><span class="n">DEFAULT_TCP_LOGGING_PORT</span><span class="p">)</span>
<span class="c1"># don't bother with a formatter, since a socket handler sends the event as</span>
<span class="c1"># an unformatted pickle</span>
<span class="n">rootLogger</span><span class="o">.</span><span class="n">addHandler</span><span class="p">(</span><span class="n">socketHandler</span><span class="p">)</span>
<span class="c1"># Now, we can log to the root logger, or any other logger. First the root...</span>
<span class="n">logging</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'Jackdaws love my big sphinx of quartz.'</span><span class="p">)</span>
<span class="c1"># Now, define a couple of other loggers which might represent areas in your</span>
<span class="c1"># application:</span>
<span class="n">logger1</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="s1">'myapp.area1'</span><span class="p">)</span>
<span class="n">logger2</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="s1">'myapp.area2'</span><span class="p">)</span>
<span class="n">logger1</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s1">'Quick zephyrs blow, vexing daft Jim.'</span><span class="p">)</span>
<span class="n">logger1</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'How quickly daft jumping zebras vex.'</span><span class="p">)</span>
<span class="n">logger2</span><span class="o">.</span><span class="n">warning</span><span class="p">(</span><span class="s1">'Jail zesty vixen who grabbed pay from quack.'</span><span class="p">)</span>
<span class="n">logger2</span><span class="o">.</span><span class="n">error</span><span class="p">(</span><span class="s1">'The five boxing wizards jump quickly.'</span><span class="p">)</span>
</pre></div>
</div>
<p>At the receiving end, you can set up a receiver using the <a class="reference internal" href="../library/socketserver.html#module-socketserver" title="socketserver: A framework for network servers."><code class="xref py py-mod docutils literal"><span class="pre">socketserver</span></code></a>
module. Here is a basic working example:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">pickle</span>
<span class="kn">import</span> <span class="nn">logging</span>
<span class="kn">import</span> <span class="nn">logging.handlers</span>
<span class="kn">import</span> <span class="nn">socketserver</span>
<span class="kn">import</span> <span class="nn">struct</span>
<span class="k">class</span> <span class="nc">LogRecordStreamHandler</span><span class="p">(</span><span class="n">socketserver</span><span class="o">.</span><span class="n">StreamRequestHandler</span><span class="p">):</span>
<span class="sd">"""Handler for a streaming logging request.</span>
<span class="sd"> This basically logs the record using whatever logging policy is</span>
<span class="sd"> configured locally.</span>
<span class="sd"> """</span>
<span class="k">def</span> <span class="nf">handle</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="sd">"""</span>
<span class="sd"> Handle multiple requests - each expected to be a 4-byte length,</span>
<span class="sd"> followed by the LogRecord in pickle format. Logs the record</span>
<span class="sd"> according to whatever policy is configured locally.</span>
<span class="sd"> """</span>
<span class="k">while</span> <span class="kc">True</span><span class="p">:</span>
<span class="n">chunk</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">connection</span><span class="o">.</span><span class="n">recv</span><span class="p">(</span><span class="mi">4</span><span class="p">)</span>
<span class="k">if</span> <span class="nb">len</span><span class="p">(</span><span class="n">chunk</span><span class="p">)</span> <span class="o"><</span> <span class="mi">4</span><span class="p">:</span>
<span class="k">break</span>
<span class="n">slen</span> <span class="o">=</span> <span class="n">struct</span><span class="o">.</span><span class="n">unpack</span><span class="p">(</span><span class="s1">'>L'</span><span class="p">,</span> <span class="n">chunk</span><span class="p">)[</span><span class="mi">0</span><span class="p">]</span>
<span class="n">chunk</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">connection</span><span class="o">.</span><span class="n">recv</span><span class="p">(</span><span class="n">slen</span><span class="p">)</span>
<span class="k">while</span> <span class="nb">len</span><span class="p">(</span><span class="n">chunk</span><span class="p">)</span> <span class="o"><</span> <span class="n">slen</span><span class="p">:</span>
<span class="n">chunk</span> <span class="o">=</span> <span class="n">chunk</span> <span class="o">+</span> <span class="bp">self</span><span class="o">.</span><span class="n">connection</span><span class="o">.</span><span class="n">recv</span><span class="p">(</span><span class="n">slen</span> <span class="o">-</span> <span class="nb">len</span><span class="p">(</span><span class="n">chunk</span><span class="p">))</span>
<span class="n">obj</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">unPickle</span><span class="p">(</span><span class="n">chunk</span><span class="p">)</span>
<span class="n">record</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">makeLogRecord</span><span class="p">(</span><span class="n">obj</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">handleLogRecord</span><span class="p">(</span><span class="n">record</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">unPickle</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">data</span><span class="p">):</span>
<span class="k">return</span> <span class="n">pickle</span><span class="o">.</span><span class="n">loads</span><span class="p">(</span><span class="n">data</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">handleLogRecord</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">record</span><span class="p">):</span>
<span class="c1"># if a name is specified, we use the named logger rather than the one</span>
<span class="c1"># implied by the record.</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">server</span><span class="o">.</span><span class="n">logname</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span><span class="p">:</span>
<span class="n">name</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">server</span><span class="o">.</span><span class="n">logname</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">name</span> <span class="o">=</span> <span class="n">record</span><span class="o">.</span><span class="n">name</span>
<span class="n">logger</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="n">name</span><span class="p">)</span>
<span class="c1"># N.B. EVERY record gets logged. This is because Logger.handle</span>
<span class="c1"># is normally called AFTER logger-level filtering. If you want</span>
<span class="c1"># to do filtering, do it at the client end to save wasting</span>
<span class="c1"># cycles and network bandwidth!</span>
<span class="n">logger</span><span class="o">.</span><span class="n">handle</span><span class="p">(</span><span class="n">record</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">LogRecordSocketReceiver</span><span class="p">(</span><span class="n">socketserver</span><span class="o">.</span><span class="n">ThreadingTCPServer</span><span class="p">):</span>
<span class="sd">"""</span>
<span class="sd"> Simple TCP socket-based logging receiver suitable for testing.</span>
<span class="sd"> """</span>
<span class="n">allow_reuse_address</span> <span class="o">=</span> <span class="kc">True</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">host</span><span class="o">=</span><span class="s1">'localhost'</span><span class="p">,</span>
<span class="n">port</span><span class="o">=</span><span class="n">logging</span><span class="o">.</span><span class="n">handlers</span><span class="o">.</span><span class="n">DEFAULT_TCP_LOGGING_PORT</span><span class="p">,</span>
<span class="n">handler</span><span class="o">=</span><span class="n">LogRecordStreamHandler</span><span class="p">):</span>
<span class="n">socketserver</span><span class="o">.</span><span class="n">ThreadingTCPServer</span><span class="o">.</span><span class="n">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="p">(</span><span class="n">host</span><span class="p">,</span> <span class="n">port</span><span class="p">),</span> <span class="n">handler</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">abort</span> <span class="o">=</span> <span class="mi">0</span>
<span class="bp">self</span><span class="o">.</span><span class="n">timeout</span> <span class="o">=</span> <span class="mi">1</span>
<span class="bp">self</span><span class="o">.</span><span class="n">logname</span> <span class="o">=</span> <span class="kc">None</span>
<span class="k">def</span> <span class="nf">serve_until_stopped</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="kn">import</span> <span class="nn">select</span>
<span class="n">abort</span> <span class="o">=</span> <span class="mi">0</span>
<span class="k">while</span> <span class="ow">not</span> <span class="n">abort</span><span class="p">:</span>
<span class="n">rd</span><span class="p">,</span> <span class="n">wr</span><span class="p">,</span> <span class="n">ex</span> <span class="o">=</span> <span class="n">select</span><span class="o">.</span><span class="n">select</span><span class="p">([</span><span class="bp">self</span><span class="o">.</span><span class="n">socket</span><span class="o">.</span><span class="n">fileno</span><span class="p">()],</span>
<span class="p">[],</span> <span class="p">[],</span>
<span class="bp">self</span><span class="o">.</span><span class="n">timeout</span><span class="p">)</span>
<span class="k">if</span> <span class="n">rd</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">handle_request</span><span class="p">()</span>
<span class="n">abort</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">abort</span>
<span class="k">def</span> <span class="nf">main</span><span class="p">():</span>
<span class="n">logging</span><span class="o">.</span><span class="n">basicConfig</span><span class="p">(</span>
<span class="nb">format</span><span class="o">=</span><span class="s1">'</span><span class="si">%(relativeCreated)5d</span><span class="s1"> </span><span class="si">%(name)-15s</span><span class="s1"> </span><span class="si">%(levelname)-8s</span><span class="s1"> </span><span class="si">%(message)s</span><span class="s1">'</span><span class="p">)</span>
<span class="n">tcpserver</span> <span class="o">=</span> <span class="n">LogRecordSocketReceiver</span><span class="p">()</span>
<span class="nb">print</span><span class="p">(</span><span class="s1">'About to start TCP server...'</span><span class="p">)</span>
<span class="n">tcpserver</span><span class="o">.</span><span class="n">serve_until_stopped</span><span class="p">()</span>
<span class="k">if</span> <span class="n">__name__</span> <span class="o">==</span> <span class="s1">'__main__'</span><span class="p">:</span>
<span class="n">main</span><span class="p">()</span>
</pre></div>
</div>
<p>First run the server, and then the client. On the client side, nothing is
printed on the console; on the server side, you should see something like:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">About</span> <span class="n">to</span> <span class="n">start</span> <span class="n">TCP</span> <span class="n">server</span><span class="o">...</span>
<span class="mi">59</span> <span class="n">root</span> <span class="n">INFO</span> <span class="n">Jackdaws</span> <span class="n">love</span> <span class="n">my</span> <span class="n">big</span> <span class="n">sphinx</span> <span class="n">of</span> <span class="n">quartz</span><span class="o">.</span>
<span class="mi">59</span> <span class="n">myapp</span><span class="o">.</span><span class="n">area1</span> <span class="n">DEBUG</span> <span class="n">Quick</span> <span class="n">zephyrs</span> <span class="n">blow</span><span class="p">,</span> <span class="n">vexing</span> <span class="n">daft</span> <span class="n">Jim</span><span class="o">.</span>
<span class="mi">69</span> <span class="n">myapp</span><span class="o">.</span><span class="n">area1</span> <span class="n">INFO</span> <span class="n">How</span> <span class="n">quickly</span> <span class="n">daft</span> <span class="n">jumping</span> <span class="n">zebras</span> <span class="n">vex</span><span class="o">.</span>
<span class="mi">69</span> <span class="n">myapp</span><span class="o">.</span><span class="n">area2</span> <span class="n">WARNING</span> <span class="n">Jail</span> <span class="n">zesty</span> <span class="n">vixen</span> <span class="n">who</span> <span class="n">grabbed</span> <span class="n">pay</span> <span class="kn">from</span> <span class="nn">quack.</span>
<span class="mi">69</span> <span class="n">myapp</span><span class="o">.</span><span class="n">area2</span> <span class="n">ERROR</span> <span class="n">The</span> <span class="n">five</span> <span class="n">boxing</span> <span class="n">wizards</span> <span class="n">jump</span> <span class="n">quickly</span><span class="o">.</span>
</pre></div>
</div>
<p>Note that there are some security issues with pickle in some scenarios. If
these affect you, you can use an alternative serialization scheme by overriding
the <code class="xref py py-meth docutils literal"><span class="pre">makePickle()</span></code> method and implementing your
alternative there, as well as adapting the above script to use your alternative
serialization.</p>
</div>
<div class="section" id="adding-contextual-information-to-your-logging-output">
<span id="context-info"></span><h2>Adding contextual information to your logging output<a class="headerlink" href="#adding-contextual-information-to-your-logging-output" title="Permalink to this headline">¶</a></h2>
<p>Sometimes you want logging output to contain contextual information in
addition to the parameters passed to the logging call. For example, in a
networked application, it may be desirable to log client-specific information
in the log (e.g. remote client’s username, or IP address). Although you could
use the <em>extra</em> parameter to achieve this, it’s not always convenient to pass
the information in this way. While it might be tempting to create
<code class="xref py py-class docutils literal"><span class="pre">Logger</span></code> instances on a per-connection basis, this is not a good idea
because these instances are not garbage collected. While this is not a problem
in practice, when the number of <code class="xref py py-class docutils literal"><span class="pre">Logger</span></code> instances is dependent on the
level of granularity you want to use in logging an application, it could
be hard to manage if the number of <code class="xref py py-class docutils literal"><span class="pre">Logger</span></code> instances becomes
effectively unbounded.</p>
<div class="section" id="using-loggeradapters-to-impart-contextual-information">
<h3>Using LoggerAdapters to impart contextual information<a class="headerlink" href="#using-loggeradapters-to-impart-contextual-information" title="Permalink to this headline">¶</a></h3>
<p>An easy way in which you can pass contextual information to be output along
with logging event information is to use the <code class="xref py py-class docutils literal"><span class="pre">LoggerAdapter</span></code> class.
This class is designed to look like a <code class="xref py py-class docutils literal"><span class="pre">Logger</span></code>, so that you can call
<code class="xref py py-meth docutils literal"><span class="pre">debug()</span></code>, <code class="xref py py-meth docutils literal"><span class="pre">info()</span></code>, <code class="xref py py-meth docutils literal"><span class="pre">warning()</span></code>, <code class="xref py py-meth docutils literal"><span class="pre">error()</span></code>,
<code class="xref py py-meth docutils literal"><span class="pre">exception()</span></code>, <code class="xref py py-meth docutils literal"><span class="pre">critical()</span></code> and <code class="xref py py-meth docutils literal"><span class="pre">log()</span></code>. These methods have the
same signatures as their counterparts in <code class="xref py py-class docutils literal"><span class="pre">Logger</span></code>, so you can use the
two types of instances interchangeably.</p>
<p>When you create an instance of <code class="xref py py-class docutils literal"><span class="pre">LoggerAdapter</span></code>, you pass it a
<code class="xref py py-class docutils literal"><span class="pre">Logger</span></code> instance and a dict-like object which contains your contextual
information. When you call one of the logging methods on an instance of
<code class="xref py py-class docutils literal"><span class="pre">LoggerAdapter</span></code>, it delegates the call to the underlying instance of
<code class="xref py py-class docutils literal"><span class="pre">Logger</span></code> passed to its constructor, and arranges to pass the contextual
information in the delegated call. Here’s a snippet from the code of
<code class="xref py py-class docutils literal"><span class="pre">LoggerAdapter</span></code>:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">debug</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">msg</span><span class="p">,</span> <span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span>
<span class="sd">"""</span>
<span class="sd"> Delegate a debug call to the underlying logger, after adding</span>
<span class="sd"> contextual information from this adapter instance.</span>
<span class="sd"> """</span>
<span class="n">msg</span><span class="p">,</span> <span class="n">kwargs</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">process</span><span class="p">(</span><span class="n">msg</span><span class="p">,</span> <span class="n">kwargs</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">logger</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="n">msg</span><span class="p">,</span> <span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">)</span>
</pre></div>
</div>
<p>The <code class="xref py py-meth docutils literal"><span class="pre">process()</span></code> method of <code class="xref py py-class docutils literal"><span class="pre">LoggerAdapter</span></code> is where the
contextual information is added to the logging output. It’s passed the message
and keyword arguments of the logging call, and it passes back (potentially)
modified versions of these to use in the call to the underlying logger. The
default implementation of this method leaves the message alone, but inserts
an ‘extra’ key in the keyword argument whose value is the dict-like object
passed to the constructor. Of course, if you had passed an ‘extra’ keyword
argument in the call to the adapter, it will be silently overwritten.</p>
<p>The advantage of using ‘extra’ is that the values in the dict-like object are
merged into the <code class="xref py py-class docutils literal"><span class="pre">LogRecord</span></code> instance’s __dict__, allowing you to use
customized strings with your <code class="xref py py-class docutils literal"><span class="pre">Formatter</span></code> instances which know about
the keys of the dict-like object. If you need a different method, e.g. if you
want to prepend or append the contextual information to the message string,
you just need to subclass <code class="xref py py-class docutils literal"><span class="pre">LoggerAdapter</span></code> and override
<code class="xref py py-meth docutils literal"><span class="pre">process()</span></code> to do what you need. Here is a simple example:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">CustomAdapter</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">LoggerAdapter</span><span class="p">):</span>
<span class="sd">"""</span>
<span class="sd"> This example adapter expects the passed in dict-like object to have a</span>
<span class="sd"> 'connid' key, whose value in brackets is prepended to the log message.</span>
<span class="sd"> """</span>
<span class="k">def</span> <span class="nf">process</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">msg</span><span class="p">,</span> <span class="n">kwargs</span><span class="p">):</span>
<span class="k">return</span> <span class="s1">'[</span><span class="si">%s</span><span class="s1">] </span><span class="si">%s</span><span class="s1">'</span> <span class="o">%</span> <span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">extra</span><span class="p">[</span><span class="s1">'connid'</span><span class="p">],</span> <span class="n">msg</span><span class="p">),</span> <span class="n">kwargs</span>
</pre></div>
</div>
<p>which you can use like this:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">logger</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="n">__name__</span><span class="p">)</span>
<span class="n">adapter</span> <span class="o">=</span> <span class="n">CustomAdapter</span><span class="p">(</span><span class="n">logger</span><span class="p">,</span> <span class="p">{</span><span class="s1">'connid'</span><span class="p">:</span> <span class="n">some_conn_id</span><span class="p">})</span>
</pre></div>
</div>
<p>Then any events that you log to the adapter will have the value of
<code class="docutils literal"><span class="pre">some_conn_id</span></code> prepended to the log messages.</p>
<div class="section" id="using-objects-other-than-dicts-to-pass-contextual-information">
<h4>Using objects other than dicts to pass contextual information<a class="headerlink" href="#using-objects-other-than-dicts-to-pass-contextual-information" title="Permalink to this headline">¶</a></h4>
<p>You don’t need to pass an actual dict to a <code class="xref py py-class docutils literal"><span class="pre">LoggerAdapter</span></code> - you could
pass an instance of a class which implements <code class="docutils literal"><span class="pre">__getitem__</span></code> and <code class="docutils literal"><span class="pre">__iter__</span></code> so
that it looks like a dict to logging. This would be useful if you want to
generate values dynamically (whereas the values in a dict would be constant).</p>
</div>
</div>
<div class="section" id="using-filters-to-impart-contextual-information">
<span id="filters-contextual"></span><h3>Using Filters to impart contextual information<a class="headerlink" href="#using-filters-to-impart-contextual-information" title="Permalink to this headline">¶</a></h3>
<p>You can also add contextual information to log output using a user-defined
<code class="xref py py-class docutils literal"><span class="pre">Filter</span></code>. <code class="docutils literal"><span class="pre">Filter</span></code> instances are allowed to modify the <code class="docutils literal"><span class="pre">LogRecords</span></code>
passed to them, including adding additional attributes which can then be output
using a suitable format string, or if needed a custom <code class="xref py py-class docutils literal"><span class="pre">Formatter</span></code>.</p>
<p>For example in a web application, the request being processed (or at least,
the interesting parts of it) can be stored in a threadlocal
(<a class="reference internal" href="../library/threading.html#threading.local" title="threading.local"><code class="xref py py-class docutils literal"><span class="pre">threading.local</span></code></a>) variable, and then accessed from a <code class="docutils literal"><span class="pre">Filter</span></code> to
add, say, information from the request - say, the remote IP address and remote
user’s username - to the <code class="docutils literal"><span class="pre">LogRecord</span></code>, using the attribute names ‘ip’ and
‘user’ as in the <code class="docutils literal"><span class="pre">LoggerAdapter</span></code> example above. In that case, the same format
string can be used to get similar output to that shown above. Here’s an example
script:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">logging</span>
<span class="kn">from</span> <span class="nn">random</span> <span class="k">import</span> <span class="n">choice</span>
<span class="k">class</span> <span class="nc">ContextFilter</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">Filter</span><span class="p">):</span>
<span class="sd">"""</span>
<span class="sd"> This is a filter which injects contextual information into the log.</span>
<span class="sd"> Rather than use actual contextual information, we just use random</span>
<span class="sd"> data in this demo.</span>
<span class="sd"> """</span>
<span class="n">USERS</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'jim'</span><span class="p">,</span> <span class="s1">'fred'</span><span class="p">,</span> <span class="s1">'sheila'</span><span class="p">]</span>
<span class="n">IPS</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'123.231.231.123'</span><span class="p">,</span> <span class="s1">'127.0.0.1'</span><span class="p">,</span> <span class="s1">'192.168.0.1'</span><span class="p">]</span>
<span class="k">def</span> <span class="nf">filter</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">record</span><span class="p">):</span>
<span class="n">record</span><span class="o">.</span><span class="n">ip</span> <span class="o">=</span> <span class="n">choice</span><span class="p">(</span><span class="n">ContextFilter</span><span class="o">.</span><span class="n">IPS</span><span class="p">)</span>
<span class="n">record</span><span class="o">.</span><span class="n">user</span> <span class="o">=</span> <span class="n">choice</span><span class="p">(</span><span class="n">ContextFilter</span><span class="o">.</span><span class="n">USERS</span><span class="p">)</span>
<span class="k">return</span> <span class="kc">True</span>
<span class="k">if</span> <span class="n">__name__</span> <span class="o">==</span> <span class="s1">'__main__'</span><span class="p">:</span>
<span class="n">levels</span> <span class="o">=</span> <span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">,</span> <span class="n">logging</span><span class="o">.</span><span class="n">INFO</span><span class="p">,</span> <span class="n">logging</span><span class="o">.</span><span class="n">WARNING</span><span class="p">,</span> <span class="n">logging</span><span class="o">.</span><span class="n">ERROR</span><span class="p">,</span> <span class="n">logging</span><span class="o">.</span><span class="n">CRITICAL</span><span class="p">)</span>
<span class="n">logging</span><span class="o">.</span><span class="n">basicConfig</span><span class="p">(</span><span class="n">level</span><span class="o">=</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">,</span>
<span class="nb">format</span><span class="o">=</span><span class="s1">'</span><span class="si">%(asctime)-15s</span><span class="s1"> </span><span class="si">%(name)-5s</span><span class="s1"> </span><span class="si">%(levelname)-8s</span><span class="s1"> IP: </span><span class="si">%(ip)-15s</span><span class="s1"> User: </span><span class="si">%(user)-8s</span><span class="s1"> </span><span class="si">%(message)s</span><span class="s1">'</span><span class="p">)</span>
<span class="n">a1</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="s1">'a.b.c'</span><span class="p">)</span>
<span class="n">a2</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="s1">'d.e.f'</span><span class="p">)</span>
<span class="n">f</span> <span class="o">=</span> <span class="n">ContextFilter</span><span class="p">()</span>
<span class="n">a1</span><span class="o">.</span><span class="n">addFilter</span><span class="p">(</span><span class="n">f</span><span class="p">)</span>
<span class="n">a2</span><span class="o">.</span><span class="n">addFilter</span><span class="p">(</span><span class="n">f</span><span class="p">)</span>
<span class="n">a1</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s1">'A debug message'</span><span class="p">)</span>
<span class="n">a1</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'An info message with </span><span class="si">%s</span><span class="s1">'</span><span class="p">,</span> <span class="s1">'some parameters'</span><span class="p">)</span>
<span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">):</span>
<span class="n">lvl</span> <span class="o">=</span> <span class="n">choice</span><span class="p">(</span><span class="n">levels</span><span class="p">)</span>
<span class="n">lvlname</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLevelName</span><span class="p">(</span><span class="n">lvl</span><span class="p">)</span>
<span class="n">a2</span><span class="o">.</span><span class="n">log</span><span class="p">(</span><span class="n">lvl</span><span class="p">,</span> <span class="s1">'A message at </span><span class="si">%s</span><span class="s1"> level with </span><span class="si">%d</span><span class="s1"> </span><span class="si">%s</span><span class="s1">'</span><span class="p">,</span> <span class="n">lvlname</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="s1">'parameters'</span><span class="p">)</span>
</pre></div>
</div>
<p>which, when run, produces something like:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="mi">2010</span><span class="o">-</span><span class="mi">09</span><span class="o">-</span><span class="mi">06</span> <span class="mi">22</span><span class="p">:</span><span class="mi">38</span><span class="p">:</span><span class="mi">15</span><span class="p">,</span><span class="mi">292</span> <span class="n">a</span><span class="o">.</span><span class="n">b</span><span class="o">.</span><span class="n">c</span> <span class="n">DEBUG</span> <span class="n">IP</span><span class="p">:</span> <span class="mf">123.231</span><span class="o">.</span><span class="mf">231.123</span> <span class="n">User</span><span class="p">:</span> <span class="n">fred</span> <span class="n">A</span> <span class="n">debug</span> <span class="n">message</span>
<span class="mi">2010</span><span class="o">-</span><span class="mi">09</span><span class="o">-</span><span class="mi">06</span> <span class="mi">22</span><span class="p">:</span><span class="mi">38</span><span class="p">:</span><span class="mi">15</span><span class="p">,</span><span class="mi">300</span> <span class="n">a</span><span class="o">.</span><span class="n">b</span><span class="o">.</span><span class="n">c</span> <span class="n">INFO</span> <span class="n">IP</span><span class="p">:</span> <span class="mf">192.168</span><span class="o">.</span><span class="mf">0.1</span> <span class="n">User</span><span class="p">:</span> <span class="n">sheila</span> <span class="n">An</span> <span class="n">info</span> <span class="n">message</span> <span class="k">with</span> <span class="n">some</span> <span class="n">parameters</span>
<span class="mi">2010</span><span class="o">-</span><span class="mi">09</span><span class="o">-</span><span class="mi">06</span> <span class="mi">22</span><span class="p">:</span><span class="mi">38</span><span class="p">:</span><span class="mi">15</span><span class="p">,</span><span class="mi">300</span> <span class="n">d</span><span class="o">.</span><span class="n">e</span><span class="o">.</span><span class="n">f</span> <span class="n">CRITICAL</span> <span class="n">IP</span><span class="p">:</span> <span class="mf">127.0</span><span class="o">.</span><span class="mf">0.1</span> <span class="n">User</span><span class="p">:</span> <span class="n">sheila</span> <span class="n">A</span> <span class="n">message</span> <span class="n">at</span> <span class="n">CRITICAL</span> <span class="n">level</span> <span class="k">with</span> <span class="mi">2</span> <span class="n">parameters</span>
<span class="mi">2010</span><span class="o">-</span><span class="mi">09</span><span class="o">-</span><span class="mi">06</span> <span class="mi">22</span><span class="p">:</span><span class="mi">38</span><span class="p">:</span><span class="mi">15</span><span class="p">,</span><span class="mi">300</span> <span class="n">d</span><span class="o">.</span><span class="n">e</span><span class="o">.</span><span class="n">f</span> <span class="n">ERROR</span> <span class="n">IP</span><span class="p">:</span> <span class="mf">127.0</span><span class="o">.</span><span class="mf">0.1</span> <span class="n">User</span><span class="p">:</span> <span class="n">jim</span> <span class="n">A</span> <span class="n">message</span> <span class="n">at</span> <span class="n">ERROR</span> <span class="n">level</span> <span class="k">with</span> <span class="mi">2</span> <span class="n">parameters</span>
<span class="mi">2010</span><span class="o">-</span><span class="mi">09</span><span class="o">-</span><span class="mi">06</span> <span class="mi">22</span><span class="p">:</span><span class="mi">38</span><span class="p">:</span><span class="mi">15</span><span class="p">,</span><span class="mi">300</span> <span class="n">d</span><span class="o">.</span><span class="n">e</span><span class="o">.</span><span class="n">f</span> <span class="n">DEBUG</span> <span class="n">IP</span><span class="p">:</span> <span class="mf">127.0</span><span class="o">.</span><span class="mf">0.1</span> <span class="n">User</span><span class="p">:</span> <span class="n">sheila</span> <span class="n">A</span> <span class="n">message</span> <span class="n">at</span> <span class="n">DEBUG</span> <span class="n">level</span> <span class="k">with</span> <span class="mi">2</span> <span class="n">parameters</span>
<span class="mi">2010</span><span class="o">-</span><span class="mi">09</span><span class="o">-</span><span class="mi">06</span> <span class="mi">22</span><span class="p">:</span><span class="mi">38</span><span class="p">:</span><span class="mi">15</span><span class="p">,</span><span class="mi">300</span> <span class="n">d</span><span class="o">.</span><span class="n">e</span><span class="o">.</span><span class="n">f</span> <span class="n">ERROR</span> <span class="n">IP</span><span class="p">:</span> <span class="mf">123.231</span><span class="o">.</span><span class="mf">231.123</span> <span class="n">User</span><span class="p">:</span> <span class="n">fred</span> <span class="n">A</span> <span class="n">message</span> <span class="n">at</span> <span class="n">ERROR</span> <span class="n">level</span> <span class="k">with</span> <span class="mi">2</span> <span class="n">parameters</span>
<span class="mi">2010</span><span class="o">-</span><span class="mi">09</span><span class="o">-</span><span class="mi">06</span> <span class="mi">22</span><span class="p">:</span><span class="mi">38</span><span class="p">:</span><span class="mi">15</span><span class="p">,</span><span class="mi">300</span> <span class="n">d</span><span class="o">.</span><span class="n">e</span><span class="o">.</span><span class="n">f</span> <span class="n">CRITICAL</span> <span class="n">IP</span><span class="p">:</span> <span class="mf">192.168</span><span class="o">.</span><span class="mf">0.1</span> <span class="n">User</span><span class="p">:</span> <span class="n">jim</span> <span class="n">A</span> <span class="n">message</span> <span class="n">at</span> <span class="n">CRITICAL</span> <span class="n">level</span> <span class="k">with</span> <span class="mi">2</span> <span class="n">parameters</span>
<span class="mi">2010</span><span class="o">-</span><span class="mi">09</span><span class="o">-</span><span class="mi">06</span> <span class="mi">22</span><span class="p">:</span><span class="mi">38</span><span class="p">:</span><span class="mi">15</span><span class="p">,</span><span class="mi">300</span> <span class="n">d</span><span class="o">.</span><span class="n">e</span><span class="o">.</span><span class="n">f</span> <span class="n">CRITICAL</span> <span class="n">IP</span><span class="p">:</span> <span class="mf">127.0</span><span class="o">.</span><span class="mf">0.1</span> <span class="n">User</span><span class="p">:</span> <span class="n">sheila</span> <span class="n">A</span> <span class="n">message</span> <span class="n">at</span> <span class="n">CRITICAL</span> <span class="n">level</span> <span class="k">with</span> <span class="mi">2</span> <span class="n">parameters</span>
<span class="mi">2010</span><span class="o">-</span><span class="mi">09</span><span class="o">-</span><span class="mi">06</span> <span class="mi">22</span><span class="p">:</span><span class="mi">38</span><span class="p">:</span><span class="mi">15</span><span class="p">,</span><span class="mi">300</span> <span class="n">d</span><span class="o">.</span><span class="n">e</span><span class="o">.</span><span class="n">f</span> <span class="n">DEBUG</span> <span class="n">IP</span><span class="p">:</span> <span class="mf">192.168</span><span class="o">.</span><span class="mf">0.1</span> <span class="n">User</span><span class="p">:</span> <span class="n">jim</span> <span class="n">A</span> <span class="n">message</span> <span class="n">at</span> <span class="n">DEBUG</span> <span class="n">level</span> <span class="k">with</span> <span class="mi">2</span> <span class="n">parameters</span>
<span class="mi">2010</span><span class="o">-</span><span class="mi">09</span><span class="o">-</span><span class="mi">06</span> <span class="mi">22</span><span class="p">:</span><span class="mi">38</span><span class="p">:</span><span class="mi">15</span><span class="p">,</span><span class="mi">301</span> <span class="n">d</span><span class="o">.</span><span class="n">e</span><span class="o">.</span><span class="n">f</span> <span class="n">ERROR</span> <span class="n">IP</span><span class="p">:</span> <span class="mf">127.0</span><span class="o">.</span><span class="mf">0.1</span> <span class="n">User</span><span class="p">:</span> <span class="n">sheila</span> <span class="n">A</span> <span class="n">message</span> <span class="n">at</span> <span class="n">ERROR</span> <span class="n">level</span> <span class="k">with</span> <span class="mi">2</span> <span class="n">parameters</span>
<span class="mi">2010</span><span class="o">-</span><span class="mi">09</span><span class="o">-</span><span class="mi">06</span> <span class="mi">22</span><span class="p">:</span><span class="mi">38</span><span class="p">:</span><span class="mi">15</span><span class="p">,</span><span class="mi">301</span> <span class="n">d</span><span class="o">.</span><span class="n">e</span><span class="o">.</span><span class="n">f</span> <span class="n">DEBUG</span> <span class="n">IP</span><span class="p">:</span> <span class="mf">123.231</span><span class="o">.</span><span class="mf">231.123</span> <span class="n">User</span><span class="p">:</span> <span class="n">fred</span> <span class="n">A</span> <span class="n">message</span> <span class="n">at</span> <span class="n">DEBUG</span> <span class="n">level</span> <span class="k">with</span> <span class="mi">2</span> <span class="n">parameters</span>
<span class="mi">2010</span><span class="o">-</span><span class="mi">09</span><span class="o">-</span><span class="mi">06</span> <span class="mi">22</span><span class="p">:</span><span class="mi">38</span><span class="p">:</span><span class="mi">15</span><span class="p">,</span><span class="mi">301</span> <span class="n">d</span><span class="o">.</span><span class="n">e</span><span class="o">.</span><span class="n">f</span> <span class="n">INFO</span> <span class="n">IP</span><span class="p">:</span> <span class="mf">123.231</span><span class="o">.</span><span class="mf">231.123</span> <span class="n">User</span><span class="p">:</span> <span class="n">fred</span> <span class="n">A</span> <span class="n">message</span> <span class="n">at</span> <span class="n">INFO</span> <span class="n">level</span> <span class="k">with</span> <span class="mi">2</span> <span class="n">parameters</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="logging-to-a-single-file-from-multiple-processes">
<span id="multiple-processes"></span><h2>Logging to a single file from multiple processes<a class="headerlink" href="#logging-to-a-single-file-from-multiple-processes" title="Permalink to this headline">¶</a></h2>
<p>Although logging is thread-safe, and logging to a single file from multiple
threads in a single process <em>is</em> supported, logging to a single file from
<em>multiple processes</em> is <em>not</em> supported, because there is no standard way to
serialize access to a single file across multiple processes in Python. If you
need to log to a single file from multiple processes, one way of doing this is
to have all the processes log to a <code class="xref py py-class docutils literal"><span class="pre">SocketHandler</span></code>, and have a
separate process which implements a socket server which reads from the socket
and logs to file. (If you prefer, you can dedicate one thread in one of the
existing processes to perform this function.)
<a class="reference internal" href="#network-logging"><span>This section</span></a> documents this approach in more detail and
includes a working socket receiver which can be used as a starting point for you
to adapt in your own applications.</p>
<p>If you are using a recent version of Python which includes the
<a class="reference internal" href="../library/multiprocessing.html#module-multiprocessing" title="multiprocessing: Process-based parallelism."><code class="xref py py-mod docutils literal"><span class="pre">multiprocessing</span></code></a> module, you could write your own handler which uses the
<a class="reference internal" href="../library/multiprocessing.html#multiprocessing.Lock" title="multiprocessing.Lock"><code class="xref py py-class docutils literal"><span class="pre">Lock</span></code></a> class from this module to serialize access to the
file from your processes. The existing <code class="xref py py-class docutils literal"><span class="pre">FileHandler</span></code> and subclasses do
not make use of <a class="reference internal" href="../library/multiprocessing.html#module-multiprocessing" title="multiprocessing: Process-based parallelism."><code class="xref py py-mod docutils literal"><span class="pre">multiprocessing</span></code></a> at present, though they may do so in the
future. Note that at present, the <a class="reference internal" href="../library/multiprocessing.html#module-multiprocessing" title="multiprocessing: Process-based parallelism."><code class="xref py py-mod docutils literal"><span class="pre">multiprocessing</span></code></a> module does not provide
working lock functionality on all platforms (see
<a class="reference external" href="https://bugs.python.org/issue3770">https://bugs.python.org/issue3770</a>).</p>
<p>Alternatively, you can use a <code class="docutils literal"><span class="pre">Queue</span></code> and a <a class="reference internal" href="../library/logging.handlers.html#logging.handlers.QueueHandler" title="logging.handlers.QueueHandler"><code class="xref py py-class docutils literal"><span class="pre">QueueHandler</span></code></a> to send
all logging events to one of the processes in your multi-process application.
The following example script demonstrates how you can do this; in the example
a separate listener process listens for events sent by other processes and logs
them according to its own logging configuration. Although the example only
demonstrates one way of doing it (for example, you may want to use a listener
thread rather than a separate listener process – the implementation would be
analogous) it does allow for completely different logging configurations for
the listener and the other processes in your application, and can be used as
the basis for code meeting your own specific requirements:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="c1"># You'll need these imports in your own code</span>
<span class="kn">import</span> <span class="nn">logging</span>
<span class="kn">import</span> <span class="nn">logging.handlers</span>
<span class="kn">import</span> <span class="nn">multiprocessing</span>
<span class="c1"># Next two import lines for this demo only</span>
<span class="kn">from</span> <span class="nn">random</span> <span class="k">import</span> <span class="n">choice</span><span class="p">,</span> <span class="n">random</span>
<span class="kn">import</span> <span class="nn">time</span>
<span class="c1">#</span>
<span class="c1"># Because you'll want to define the logging configurations for listener and workers, the</span>
<span class="c1"># listener and worker process functions take a configurer parameter which is a callable</span>
<span class="c1"># for configuring logging for that process. These functions are also passed the queue,</span>
<span class="c1"># which they use for communication.</span>
<span class="c1">#</span>
<span class="c1"># In practice, you can configure the listener however you want, but note that in this</span>
<span class="c1"># simple example, the listener does not apply level or filter logic to received records.</span>
<span class="c1"># In practice, you would probably want to do this logic in the worker processes, to avoid</span>
<span class="c1"># sending events which would be filtered out between processes.</span>
<span class="c1">#</span>
<span class="c1"># The size of the rotated files is made small so you can see the results easily.</span>
<span class="k">def</span> <span class="nf">listener_configurer</span><span class="p">():</span>
<span class="n">root</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">()</span>
<span class="n">h</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">handlers</span><span class="o">.</span><span class="n">RotatingFileHandler</span><span class="p">(</span><span class="s1">'mptest.log'</span><span class="p">,</span> <span class="s1">'a'</span><span class="p">,</span> <span class="mi">300</span><span class="p">,</span> <span class="mi">10</span><span class="p">)</span>
<span class="n">f</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">Formatter</span><span class="p">(</span><span class="s1">'</span><span class="si">%(asctime)s</span><span class="s1"> </span><span class="si">%(processName)-10s</span><span class="s1"> </span><span class="si">%(name)s</span><span class="s1"> </span><span class="si">%(levelname)-8s</span><span class="s1"> </span><span class="si">%(message)s</span><span class="s1">'</span><span class="p">)</span>
<span class="n">h</span><span class="o">.</span><span class="n">setFormatter</span><span class="p">(</span><span class="n">f</span><span class="p">)</span>
<span class="n">root</span><span class="o">.</span><span class="n">addHandler</span><span class="p">(</span><span class="n">h</span><span class="p">)</span>
<span class="c1"># This is the listener process top-level loop: wait for logging events</span>
<span class="c1"># (LogRecords)on the queue and handle them, quit when you get a None for a</span>
<span class="c1"># LogRecord.</span>
<span class="k">def</span> <span class="nf">listener_process</span><span class="p">(</span><span class="n">queue</span><span class="p">,</span> <span class="n">configurer</span><span class="p">):</span>
<span class="n">configurer</span><span class="p">()</span>
<span class="k">while</span> <span class="kc">True</span><span class="p">:</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">record</span> <span class="o">=</span> <span class="n">queue</span><span class="o">.</span><span class="n">get</span><span class="p">()</span>
<span class="k">if</span> <span class="n">record</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span> <span class="c1"># We send this as a sentinel to tell the listener to quit.</span>
<span class="k">break</span>
<span class="n">logger</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="n">record</span><span class="o">.</span><span class="n">name</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">handle</span><span class="p">(</span><span class="n">record</span><span class="p">)</span> <span class="c1"># No level or filter logic applied - just do it!</span>
<span class="k">except</span> <span class="ne">Exception</span><span class="p">:</span>
<span class="kn">import</span> <span class="nn">sys</span><span class="o">,</span> <span class="nn">traceback</span>
<span class="nb">print</span><span class="p">(</span><span class="s1">'Whoops! Problem:'</span><span class="p">,</span> <span class="n">file</span><span class="o">=</span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">)</span>
<span class="n">traceback</span><span class="o">.</span><span class="n">print_exc</span><span class="p">(</span><span class="n">file</span><span class="o">=</span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">)</span>
<span class="c1"># Arrays used for random selections in this demo</span>
<span class="n">LEVELS</span> <span class="o">=</span> <span class="p">[</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">,</span> <span class="n">logging</span><span class="o">.</span><span class="n">INFO</span><span class="p">,</span> <span class="n">logging</span><span class="o">.</span><span class="n">WARNING</span><span class="p">,</span>
<span class="n">logging</span><span class="o">.</span><span class="n">ERROR</span><span class="p">,</span> <span class="n">logging</span><span class="o">.</span><span class="n">CRITICAL</span><span class="p">]</span>
<span class="n">LOGGERS</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'a.b.c'</span><span class="p">,</span> <span class="s1">'d.e.f'</span><span class="p">]</span>
<span class="n">MESSAGES</span> <span class="o">=</span> <span class="p">[</span>
<span class="s1">'Random message #1'</span><span class="p">,</span>
<span class="s1">'Random message #2'</span><span class="p">,</span>
<span class="s1">'Random message #3'</span><span class="p">,</span>
<span class="p">]</span>
<span class="c1"># The worker configuration is done at the start of the worker process run.</span>
<span class="c1"># Note that on Windows you can't rely on fork semantics, so each process</span>
<span class="c1"># will run the logging configuration code when it starts.</span>
<span class="k">def</span> <span class="nf">worker_configurer</span><span class="p">(</span><span class="n">queue</span><span class="p">):</span>
<span class="n">h</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">handlers</span><span class="o">.</span><span class="n">QueueHandler</span><span class="p">(</span><span class="n">queue</span><span class="p">)</span> <span class="c1"># Just the one handler needed</span>
<span class="n">root</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">()</span>
<span class="n">root</span><span class="o">.</span><span class="n">addHandler</span><span class="p">(</span><span class="n">h</span><span class="p">)</span>
<span class="c1"># send all messages, for demo; no other level or filter logic applied.</span>
<span class="n">root</span><span class="o">.</span><span class="n">setLevel</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">)</span>
<span class="c1"># This is the worker process top-level loop, which just logs ten events with</span>
<span class="c1"># random intervening delays before terminating.</span>
<span class="c1"># The print messages are just so you know it's doing something!</span>
<span class="k">def</span> <span class="nf">worker_process</span><span class="p">(</span><span class="n">queue</span><span class="p">,</span> <span class="n">configurer</span><span class="p">):</span>
<span class="n">configurer</span><span class="p">(</span><span class="n">queue</span><span class="p">)</span>
<span class="n">name</span> <span class="o">=</span> <span class="n">multiprocessing</span><span class="o">.</span><span class="n">current_process</span><span class="p">()</span><span class="o">.</span><span class="n">name</span>
<span class="nb">print</span><span class="p">(</span><span class="s1">'Worker started: </span><span class="si">%s</span><span class="s1">'</span> <span class="o">%</span> <span class="n">name</span><span class="p">)</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">):</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="n">random</span><span class="p">())</span>
<span class="n">logger</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="n">choice</span><span class="p">(</span><span class="n">LOGGERS</span><span class="p">))</span>
<span class="n">level</span> <span class="o">=</span> <span class="n">choice</span><span class="p">(</span><span class="n">LEVELS</span><span class="p">)</span>
<span class="n">message</span> <span class="o">=</span> <span class="n">choice</span><span class="p">(</span><span class="n">MESSAGES</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">log</span><span class="p">(</span><span class="n">level</span><span class="p">,</span> <span class="n">message</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="s1">'Worker finished: </span><span class="si">%s</span><span class="s1">'</span> <span class="o">%</span> <span class="n">name</span><span class="p">)</span>
<span class="c1"># Here's where the demo gets orchestrated. Create the queue, create and start</span>
<span class="c1"># the listener, create ten workers and start them, wait for them to finish,</span>
<span class="c1"># then send a None to the queue to tell the listener to finish.</span>
<span class="k">def</span> <span class="nf">main</span><span class="p">():</span>
<span class="n">queue</span> <span class="o">=</span> <span class="n">multiprocessing</span><span class="o">.</span><span class="n">Queue</span><span class="p">(</span><span class="o">-</span><span class="mi">1</span><span class="p">)</span>
<span class="n">listener</span> <span class="o">=</span> <span class="n">multiprocessing</span><span class="o">.</span><span class="n">Process</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">listener_process</span><span class="p">,</span>
<span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">queue</span><span class="p">,</span> <span class="n">listener_configurer</span><span class="p">))</span>
<span class="n">listener</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="n">workers</span> <span class="o">=</span> <span class="p">[]</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">):</span>
<span class="n">worker</span> <span class="o">=</span> <span class="n">multiprocessing</span><span class="o">.</span><span class="n">Process</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">worker_process</span><span class="p">,</span>
<span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">queue</span><span class="p">,</span> <span class="n">worker_configurer</span><span class="p">))</span>
<span class="n">workers</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">worker</span><span class="p">)</span>
<span class="n">worker</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="k">for</span> <span class="n">w</span> <span class="ow">in</span> <span class="n">workers</span><span class="p">:</span>
<span class="n">w</span><span class="o">.</span><span class="n">join</span><span class="p">()</span>
<span class="n">queue</span><span class="o">.</span><span class="n">put_nowait</span><span class="p">(</span><span class="kc">None</span><span class="p">)</span>
<span class="n">listener</span><span class="o">.</span><span class="n">join</span><span class="p">()</span>
<span class="k">if</span> <span class="n">__name__</span> <span class="o">==</span> <span class="s1">'__main__'</span><span class="p">:</span>
<span class="n">main</span><span class="p">()</span>
</pre></div>
</div>
<p>A variant of the above script keeps the logging in the main process, in a
separate thread:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">logging</span>
<span class="kn">import</span> <span class="nn">logging.config</span>
<span class="kn">import</span> <span class="nn">logging.handlers</span>
<span class="kn">from</span> <span class="nn">multiprocessing</span> <span class="k">import</span> <span class="n">Process</span><span class="p">,</span> <span class="n">Queue</span>
<span class="kn">import</span> <span class="nn">random</span>
<span class="kn">import</span> <span class="nn">threading</span>
<span class="kn">import</span> <span class="nn">time</span>
<span class="k">def</span> <span class="nf">logger_thread</span><span class="p">(</span><span class="n">q</span><span class="p">):</span>
<span class="k">while</span> <span class="kc">True</span><span class="p">:</span>
<span class="n">record</span> <span class="o">=</span> <span class="n">q</span><span class="o">.</span><span class="n">get</span><span class="p">()</span>
<span class="k">if</span> <span class="n">record</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
<span class="k">break</span>
<span class="n">logger</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="n">record</span><span class="o">.</span><span class="n">name</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">handle</span><span class="p">(</span><span class="n">record</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">worker_process</span><span class="p">(</span><span class="n">q</span><span class="p">):</span>
<span class="n">qh</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">handlers</span><span class="o">.</span><span class="n">QueueHandler</span><span class="p">(</span><span class="n">q</span><span class="p">)</span>
<span class="n">root</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">()</span>
<span class="n">root</span><span class="o">.</span><span class="n">setLevel</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">)</span>
<span class="n">root</span><span class="o">.</span><span class="n">addHandler</span><span class="p">(</span><span class="n">qh</span><span class="p">)</span>
<span class="n">levels</span> <span class="o">=</span> <span class="p">[</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">,</span> <span class="n">logging</span><span class="o">.</span><span class="n">INFO</span><span class="p">,</span> <span class="n">logging</span><span class="o">.</span><span class="n">WARNING</span><span class="p">,</span> <span class="n">logging</span><span class="o">.</span><span class="n">ERROR</span><span class="p">,</span>
<span class="n">logging</span><span class="o">.</span><span class="n">CRITICAL</span><span class="p">]</span>
<span class="n">loggers</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'foo'</span><span class="p">,</span> <span class="s1">'foo.bar'</span><span class="p">,</span> <span class="s1">'foo.bar.baz'</span><span class="p">,</span>
<span class="s1">'spam'</span><span class="p">,</span> <span class="s1">'spam.ham'</span><span class="p">,</span> <span class="s1">'spam.ham.eggs'</span><span class="p">]</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">100</span><span class="p">):</span>
<span class="n">lvl</span> <span class="o">=</span> <span class="n">random</span><span class="o">.</span><span class="n">choice</span><span class="p">(</span><span class="n">levels</span><span class="p">)</span>
<span class="n">logger</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="n">random</span><span class="o">.</span><span class="n">choice</span><span class="p">(</span><span class="n">loggers</span><span class="p">))</span>
<span class="n">logger</span><span class="o">.</span><span class="n">log</span><span class="p">(</span><span class="n">lvl</span><span class="p">,</span> <span class="s1">'Message no. </span><span class="si">%d</span><span class="s1">'</span><span class="p">,</span> <span class="n">i</span><span class="p">)</span>
<span class="k">if</span> <span class="n">__name__</span> <span class="o">==</span> <span class="s1">'__main__'</span><span class="p">:</span>
<span class="n">q</span> <span class="o">=</span> <span class="n">Queue</span><span class="p">()</span>
<span class="n">d</span> <span class="o">=</span> <span class="p">{</span>
<span class="s1">'version'</span><span class="p">:</span> <span class="mi">1</span><span class="p">,</span>
<span class="s1">'formatters'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'detailed'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'class'</span><span class="p">:</span> <span class="s1">'logging.Formatter'</span><span class="p">,</span>
<span class="s1">'format'</span><span class="p">:</span> <span class="s1">'</span><span class="si">%(asctime)s</span><span class="s1"> </span><span class="si">%(name)-15s</span><span class="s1"> </span><span class="si">%(levelname)-8s</span><span class="s1"> </span><span class="si">%(processName)-10s</span><span class="s1"> </span><span class="si">%(message)s</span><span class="s1">'</span>
<span class="p">}</span>
<span class="p">},</span>
<span class="s1">'handlers'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'console'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'class'</span><span class="p">:</span> <span class="s1">'logging.StreamHandler'</span><span class="p">,</span>
<span class="s1">'level'</span><span class="p">:</span> <span class="s1">'INFO'</span><span class="p">,</span>
<span class="p">},</span>
<span class="s1">'file'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'class'</span><span class="p">:</span> <span class="s1">'logging.FileHandler'</span><span class="p">,</span>
<span class="s1">'filename'</span><span class="p">:</span> <span class="s1">'mplog.log'</span><span class="p">,</span>
<span class="s1">'mode'</span><span class="p">:</span> <span class="s1">'w'</span><span class="p">,</span>
<span class="s1">'formatter'</span><span class="p">:</span> <span class="s1">'detailed'</span><span class="p">,</span>
<span class="p">},</span>
<span class="s1">'foofile'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'class'</span><span class="p">:</span> <span class="s1">'logging.FileHandler'</span><span class="p">,</span>
<span class="s1">'filename'</span><span class="p">:</span> <span class="s1">'mplog-foo.log'</span><span class="p">,</span>
<span class="s1">'mode'</span><span class="p">:</span> <span class="s1">'w'</span><span class="p">,</span>
<span class="s1">'formatter'</span><span class="p">:</span> <span class="s1">'detailed'</span><span class="p">,</span>
<span class="p">},</span>
<span class="s1">'errors'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'class'</span><span class="p">:</span> <span class="s1">'logging.FileHandler'</span><span class="p">,</span>
<span class="s1">'filename'</span><span class="p">:</span> <span class="s1">'mplog-errors.log'</span><span class="p">,</span>
<span class="s1">'mode'</span><span class="p">:</span> <span class="s1">'w'</span><span class="p">,</span>
<span class="s1">'level'</span><span class="p">:</span> <span class="s1">'ERROR'</span><span class="p">,</span>
<span class="s1">'formatter'</span><span class="p">:</span> <span class="s1">'detailed'</span><span class="p">,</span>
<span class="p">},</span>
<span class="p">},</span>
<span class="s1">'loggers'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'foo'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'handlers'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'foofile'</span><span class="p">]</span>
<span class="p">}</span>
<span class="p">},</span>
<span class="s1">'root'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'level'</span><span class="p">:</span> <span class="s1">'DEBUG'</span><span class="p">,</span>
<span class="s1">'handlers'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'console'</span><span class="p">,</span> <span class="s1">'file'</span><span class="p">,</span> <span class="s1">'errors'</span><span class="p">]</span>
<span class="p">},</span>
<span class="p">}</span>
<span class="n">workers</span> <span class="o">=</span> <span class="p">[]</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">5</span><span class="p">):</span>
<span class="n">wp</span> <span class="o">=</span> <span class="n">Process</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">worker_process</span><span class="p">,</span> <span class="n">name</span><span class="o">=</span><span class="s1">'worker </span><span class="si">%d</span><span class="s1">'</span> <span class="o">%</span> <span class="p">(</span><span class="n">i</span> <span class="o">+</span> <span class="mi">1</span><span class="p">),</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">q</span><span class="p">,))</span>
<span class="n">workers</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">wp</span><span class="p">)</span>
<span class="n">wp</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="n">logging</span><span class="o">.</span><span class="n">config</span><span class="o">.</span><span class="n">dictConfig</span><span class="p">(</span><span class="n">d</span><span class="p">)</span>
<span class="n">lp</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">logger_thread</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">q</span><span class="p">,))</span>
<span class="n">lp</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="c1"># At this point, the main process could do some useful work of its own</span>
<span class="c1"># Once it's done that, it can wait for the workers to terminate...</span>
<span class="k">for</span> <span class="n">wp</span> <span class="ow">in</span> <span class="n">workers</span><span class="p">:</span>
<span class="n">wp</span><span class="o">.</span><span class="n">join</span><span class="p">()</span>
<span class="c1"># And now tell the logging thread to finish up, too</span>
<span class="n">q</span><span class="o">.</span><span class="n">put</span><span class="p">(</span><span class="kc">None</span><span class="p">)</span>
<span class="n">lp</span><span class="o">.</span><span class="n">join</span><span class="p">()</span>
</pre></div>
</div>
<p>This variant shows how you can e.g. apply configuration for particular loggers
- e.g. the <code class="docutils literal"><span class="pre">foo</span></code> logger has a special handler which stores all events in the
<code class="docutils literal"><span class="pre">foo</span></code> subsystem in a file <code class="docutils literal"><span class="pre">mplog-foo.log</span></code>. This will be used by the logging
machinery in the main process (even though the logging events are generated in
the worker processes) to direct the messages to the appropriate destinations.</p>
</div>
<div class="section" id="using-file-rotation">
<h2>Using file rotation<a class="headerlink" href="#using-file-rotation" title="Permalink to this headline">¶</a></h2>
<p>Sometimes you want to let a log file grow to a certain size, then open a new
file and log to that. You may want to keep a certain number of these files, and
when that many files have been created, rotate the files so that the number of
files and the size of the files both remain bounded. For this usage pattern, the
logging package provides a <code class="xref py py-class docutils literal"><span class="pre">RotatingFileHandler</span></code>:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">glob</span>
<span class="kn">import</span> <span class="nn">logging</span>
<span class="kn">import</span> <span class="nn">logging.handlers</span>
<span class="n">LOG_FILENAME</span> <span class="o">=</span> <span class="s1">'logging_rotatingfile_example.out'</span>
<span class="c1"># Set up a specific logger with our desired output level</span>
<span class="n">my_logger</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="s1">'MyLogger'</span><span class="p">)</span>
<span class="n">my_logger</span><span class="o">.</span><span class="n">setLevel</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">)</span>
<span class="c1"># Add the log message handler to the logger</span>
<span class="n">handler</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">handlers</span><span class="o">.</span><span class="n">RotatingFileHandler</span><span class="p">(</span>
<span class="n">LOG_FILENAME</span><span class="p">,</span> <span class="n">maxBytes</span><span class="o">=</span><span class="mi">20</span><span class="p">,</span> <span class="n">backupCount</span><span class="o">=</span><span class="mi">5</span><span class="p">)</span>
<span class="n">my_logger</span><span class="o">.</span><span class="n">addHandler</span><span class="p">(</span><span class="n">handler</span><span class="p">)</span>
<span class="c1"># Log some messages</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">20</span><span class="p">):</span>
<span class="n">my_logger</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s1">'i = </span><span class="si">%d</span><span class="s1">'</span> <span class="o">%</span> <span class="n">i</span><span class="p">)</span>
<span class="c1"># See what files are created</span>
<span class="n">logfiles</span> <span class="o">=</span> <span class="n">glob</span><span class="o">.</span><span class="n">glob</span><span class="p">(</span><span class="s1">'</span><span class="si">%s</span><span class="s1">*'</span> <span class="o">%</span> <span class="n">LOG_FILENAME</span><span class="p">)</span>
<span class="k">for</span> <span class="n">filename</span> <span class="ow">in</span> <span class="n">logfiles</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="n">filename</span><span class="p">)</span>
</pre></div>
</div>
<p>The result should be 6 separate files, each with part of the log history for the
application:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">logging_rotatingfile_example</span><span class="o">.</span><span class="n">out</span>
<span class="n">logging_rotatingfile_example</span><span class="o">.</span><span class="n">out</span><span class="o">.</span><span class="mi">1</span>
<span class="n">logging_rotatingfile_example</span><span class="o">.</span><span class="n">out</span><span class="o">.</span><span class="mi">2</span>
<span class="n">logging_rotatingfile_example</span><span class="o">.</span><span class="n">out</span><span class="o">.</span><span class="mi">3</span>
<span class="n">logging_rotatingfile_example</span><span class="o">.</span><span class="n">out</span><span class="o">.</span><span class="mi">4</span>
<span class="n">logging_rotatingfile_example</span><span class="o">.</span><span class="n">out</span><span class="o">.</span><span class="mi">5</span>
</pre></div>
</div>
<p>The most current file is always <code class="file docutils literal"><span class="pre">logging_rotatingfile_example.out</span></code>,
and each time it reaches the size limit it is renamed with the suffix
<code class="docutils literal"><span class="pre">.1</span></code>. Each of the existing backup files is renamed to increment the suffix
(<code class="docutils literal"><span class="pre">.1</span></code> becomes <code class="docutils literal"><span class="pre">.2</span></code>, etc.) and the <code class="docutils literal"><span class="pre">.6</span></code> file is erased.</p>
<p>Obviously this example sets the log length much too small as an extreme
example. You would want to set <em>maxBytes</em> to an appropriate value.</p>
</div>
<div class="section" id="use-of-alternative-formatting-styles">
<span id="format-styles"></span><h2>Use of alternative formatting styles<a class="headerlink" href="#use-of-alternative-formatting-styles" title="Permalink to this headline">¶</a></h2>
<p>When logging was added to the Python standard library, the only way of
formatting messages with variable content was to use the %-formatting
method. Since then, Python has gained two new formatting approaches:
<a class="reference internal" href="../library/string.html#string.Template" title="string.Template"><code class="xref py py-class docutils literal"><span class="pre">string.Template</span></code></a> (added in Python 2.4) and <a class="reference internal" href="../library/stdtypes.html#str.format" title="str.format"><code class="xref py py-meth docutils literal"><span class="pre">str.format()</span></code></a>
(added in Python 2.6).</p>
<p>Logging (as of 3.2) provides improved support for these two additional
formatting styles. The <code class="xref py py-class docutils literal"><span class="pre">Formatter</span></code> class been enhanced to take an
additional, optional keyword parameter named <code class="docutils literal"><span class="pre">style</span></code>. This defaults to
<code class="docutils literal"><span class="pre">'%'</span></code>, but other possible values are <code class="docutils literal"><span class="pre">'{'</span></code> and <code class="docutils literal"><span class="pre">'$'</span></code>, which correspond
to the other two formatting styles. Backwards compatibility is maintained by
default (as you would expect), but by explicitly specifying a style parameter,
you get the ability to specify format strings which work with
<a class="reference internal" href="../library/stdtypes.html#str.format" title="str.format"><code class="xref py py-meth docutils literal"><span class="pre">str.format()</span></code></a> or <a class="reference internal" href="../library/string.html#string.Template" title="string.Template"><code class="xref py py-class docutils literal"><span class="pre">string.Template</span></code></a>. Here’s an example console
session to show the possibilities:</p>
<div class="highlight-pycon"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">logging</span>
<span class="gp">>>> </span><span class="n">root</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">root</span><span class="o">.</span><span class="n">setLevel</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">handler</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">StreamHandler</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">bf</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">Formatter</span><span class="p">(</span><span class="s1">'{asctime} {name} {levelname:8s} {message}'</span><span class="p">,</span>
<span class="gp">... </span> <span class="n">style</span><span class="o">=</span><span class="s1">'{'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">handler</span><span class="o">.</span><span class="n">setFormatter</span><span class="p">(</span><span class="n">bf</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">root</span><span class="o">.</span><span class="n">addHandler</span><span class="p">(</span><span class="n">handler</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">logger</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="s1">'foo.bar'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">logger</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s1">'This is a DEBUG message'</span><span class="p">)</span>
<span class="go">2010-10-28 15:11:55,341 foo.bar DEBUG This is a DEBUG message</span>
<span class="gp">>>> </span><span class="n">logger</span><span class="o">.</span><span class="n">critical</span><span class="p">(</span><span class="s1">'This is a CRITICAL message'</span><span class="p">)</span>
<span class="go">2010-10-28 15:12:11,526 foo.bar CRITICAL This is a CRITICAL message</span>
<span class="gp">>>> </span><span class="n">df</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">Formatter</span><span class="p">(</span><span class="s1">'$asctime $name ${levelname} $message'</span><span class="p">,</span>
<span class="gp">... </span> <span class="n">style</span><span class="o">=</span><span class="s1">'$'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">handler</span><span class="o">.</span><span class="n">setFormatter</span><span class="p">(</span><span class="n">df</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">logger</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s1">'This is a DEBUG message'</span><span class="p">)</span>
<span class="go">2010-10-28 15:13:06,924 foo.bar DEBUG This is a DEBUG message</span>
<span class="gp">>>> </span><span class="n">logger</span><span class="o">.</span><span class="n">critical</span><span class="p">(</span><span class="s1">'This is a CRITICAL message'</span><span class="p">)</span>
<span class="go">2010-10-28 15:13:11,494 foo.bar CRITICAL This is a CRITICAL message</span>
<span class="go">>>></span>
</pre></div>
</div>
<p>Note that the formatting of logging messages for final output to logs is
completely independent of how an individual logging message is constructed.
That can still use %-formatting, as shown here:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">logger</span><span class="o">.</span><span class="n">error</span><span class="p">(</span><span class="s1">'This is an</span><span class="si">%s</span><span class="s1"> </span><span class="si">%s</span><span class="s1"> </span><span class="si">%s</span><span class="s1">'</span><span class="p">,</span> <span class="s1">'other,'</span><span class="p">,</span> <span class="s1">'ERROR,'</span><span class="p">,</span> <span class="s1">'message'</span><span class="p">)</span>
<span class="go">2010-10-28 15:19:29,833 foo.bar ERROR This is another, ERROR, message</span>
<span class="go">>>></span>
</pre></div>
</div>
<p>Logging calls (<code class="docutils literal"><span class="pre">logger.debug()</span></code>, <code class="docutils literal"><span class="pre">logger.info()</span></code> etc.) only take
positional parameters for the actual logging message itself, with keyword
parameters used only for determining options for how to handle the actual
logging call (e.g. the <code class="docutils literal"><span class="pre">exc_info</span></code> keyword parameter to indicate that
traceback information should be logged, or the <code class="docutils literal"><span class="pre">extra</span></code> keyword parameter
to indicate additional contextual information to be added to the log). So
you cannot directly make logging calls using <a class="reference internal" href="../library/stdtypes.html#str.format" title="str.format"><code class="xref py py-meth docutils literal"><span class="pre">str.format()</span></code></a> or
<a class="reference internal" href="../library/string.html#string.Template" title="string.Template"><code class="xref py py-class docutils literal"><span class="pre">string.Template</span></code></a> syntax, because internally the logging package
uses %-formatting to merge the format string and the variable arguments.
There would no changing this while preserving backward compatibility, since
all logging calls which are out there in existing code will be using %-format
strings.</p>
<p>There is, however, a way that you can use {}- and $- formatting to construct
your individual log messages. Recall that for a message you can use an
arbitrary object as a message format string, and that the logging package will
call <code class="docutils literal"><span class="pre">str()</span></code> on that object to get the actual format string. Consider the
following two classes:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">BraceMessage</span><span class="p">:</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">fmt</span><span class="p">,</span> <span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">fmt</span> <span class="o">=</span> <span class="n">fmt</span>
<span class="bp">self</span><span class="o">.</span><span class="n">args</span> <span class="o">=</span> <span class="n">args</span>
<span class="bp">self</span><span class="o">.</span><span class="n">kwargs</span> <span class="o">=</span> <span class="n">kwargs</span>
<span class="k">def</span> <span class="nf">__str__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">fmt</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="o">*</span><span class="bp">self</span><span class="o">.</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="bp">self</span><span class="o">.</span><span class="n">kwargs</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">DollarMessage</span><span class="p">:</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">fmt</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">fmt</span> <span class="o">=</span> <span class="n">fmt</span>
<span class="bp">self</span><span class="o">.</span><span class="n">kwargs</span> <span class="o">=</span> <span class="n">kwargs</span>
<span class="k">def</span> <span class="nf">__str__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="kn">from</span> <span class="nn">string</span> <span class="k">import</span> <span class="n">Template</span>
<span class="k">return</span> <span class="n">Template</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">fmt</span><span class="p">)</span><span class="o">.</span><span class="n">substitute</span><span class="p">(</span><span class="o">**</span><span class="bp">self</span><span class="o">.</span><span class="n">kwargs</span><span class="p">)</span>
</pre></div>
</div>
<p>Either of these can be used in place of a format string, to allow {}- or
$-formatting to be used to build the actual “message” part which appears in the
formatted log output in place of “%(message)s” or “{message}” or “$message”.
It’s a little unwieldy to use the class names whenever you want to log
something, but it’s quite palatable if you use an alias such as __ (double
underscore — not to be confused with _, the single underscore used as a
synonym/alias for <a class="reference internal" href="../library/gettext.html#gettext.gettext" title="gettext.gettext"><code class="xref py py-func docutils literal"><span class="pre">gettext.gettext()</span></code></a> or its brethren).</p>
<p>The above classes are not included in Python, though they’re easy enough to
copy and paste into your own code. They can be used as follows (assuming that
they’re declared in a module called <code class="docutils literal"><span class="pre">wherever</span></code>):</p>
<div class="highlight-pycon"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">wherever</span> <span class="kn">import</span> <span class="n">BraceMessage</span> <span class="k">as</span> <span class="n">__</span>
<span class="gp">>>> </span><span class="k">print</span><span class="p">(</span><span class="n">__</span><span class="p">(</span><span class="s1">'Message with {0} {name}'</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="n">name</span><span class="o">=</span><span class="s1">'placeholders'</span><span class="p">))</span>
<span class="go">Message with 2 placeholders</span>
<span class="gp">>>> </span><span class="k">class</span> <span class="nc">Point</span><span class="p">:</span> <span class="k">pass</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="n">p</span> <span class="o">=</span> <span class="n">Point</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">p</span><span class="o">.</span><span class="n">x</span> <span class="o">=</span> <span class="mf">0.5</span>
<span class="gp">>>> </span><span class="n">p</span><span class="o">.</span><span class="n">y</span> <span class="o">=</span> <span class="mf">0.5</span>
<span class="gp">>>> </span><span class="k">print</span><span class="p">(</span><span class="n">__</span><span class="p">(</span><span class="s1">'Message with coordinates: ({point.x:.2f}, {point.y:.2f})'</span><span class="p">,</span>
<span class="gp">... </span> <span class="n">point</span><span class="o">=</span><span class="n">p</span><span class="p">))</span>
<span class="go">Message with coordinates: (0.50, 0.50)</span>
<span class="gp">>>> </span><span class="kn">from</span> <span class="nn">wherever</span> <span class="kn">import</span> <span class="n">DollarMessage</span> <span class="k">as</span> <span class="n">__</span>
<span class="gp">>>> </span><span class="k">print</span><span class="p">(</span><span class="n">__</span><span class="p">(</span><span class="s1">'Message with $num $what'</span><span class="p">,</span> <span class="n">num</span><span class="o">=</span><span class="mi">2</span><span class="p">,</span> <span class="n">what</span><span class="o">=</span><span class="s1">'placeholders'</span><span class="p">))</span>
<span class="go">Message with 2 placeholders</span>
<span class="go">>>></span>
</pre></div>
</div>
<p>While the above examples use <code class="docutils literal"><span class="pre">print()</span></code> to show how the formatting works, you
would of course use <code class="docutils literal"><span class="pre">logger.debug()</span></code> or similar to actually log using this
approach.</p>
<p>One thing to note is that you pay no significant performance penalty with this
approach: the actual formatting happens not when you make the logging call, but
when (and if) the logged message is actually about to be output to a log by a
handler. So the only slightly unusual thing which might trip you up is that the
parentheses go around the format string and the arguments, not just the format
string. That’s because the __ notation is just syntax sugar for a constructor
call to one of the XXXMessage classes.</p>
<p>If you prefer, you can use a <code class="xref py py-class docutils literal"><span class="pre">LoggerAdapter</span></code> to achieve a similar effect
to the above, as in the following example:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">logging</span>
<span class="k">class</span> <span class="nc">Message</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">fmt</span><span class="p">,</span> <span class="n">args</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">fmt</span> <span class="o">=</span> <span class="n">fmt</span>
<span class="bp">self</span><span class="o">.</span><span class="n">args</span> <span class="o">=</span> <span class="n">args</span>
<span class="k">def</span> <span class="nf">__str__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">fmt</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="o">*</span><span class="bp">self</span><span class="o">.</span><span class="n">args</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">StyleAdapter</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">LoggerAdapter</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">logger</span><span class="p">,</span> <span class="n">extra</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span>
<span class="nb">super</span><span class="p">(</span><span class="n">StyleAdapter</span><span class="p">,</span> <span class="bp">self</span><span class="p">)</span><span class="o">.</span><span class="n">__init__</span><span class="p">(</span><span class="n">logger</span><span class="p">,</span> <span class="n">extra</span> <span class="ow">or</span> <span class="p">{})</span>
<span class="k">def</span> <span class="nf">log</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">level</span><span class="p">,</span> <span class="n">msg</span><span class="p">,</span> <span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">isEnabledFor</span><span class="p">(</span><span class="n">level</span><span class="p">):</span>
<span class="n">msg</span><span class="p">,</span> <span class="n">kwargs</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">process</span><span class="p">(</span><span class="n">msg</span><span class="p">,</span> <span class="n">kwargs</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">logger</span><span class="o">.</span><span class="n">_log</span><span class="p">(</span><span class="n">level</span><span class="p">,</span> <span class="n">Message</span><span class="p">(</span><span class="n">msg</span><span class="p">,</span> <span class="n">args</span><span class="p">),</span> <span class="p">(),</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">)</span>
<span class="n">logger</span> <span class="o">=</span> <span class="n">StyleAdapter</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="n">__name__</span><span class="p">))</span>
<span class="k">def</span> <span class="nf">main</span><span class="p">():</span>
<span class="n">logger</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s1">'Hello, </span><span class="si">{}</span><span class="s1">'</span><span class="p">,</span> <span class="s1">'world!'</span><span class="p">)</span>
<span class="k">if</span> <span class="n">__name__</span> <span class="o">==</span> <span class="s1">'__main__'</span><span class="p">:</span>
<span class="n">logging</span><span class="o">.</span><span class="n">basicConfig</span><span class="p">(</span><span class="n">level</span><span class="o">=</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">)</span>
<span class="n">main</span><span class="p">()</span>
</pre></div>
</div>
<p>The above script should log the message <code class="docutils literal"><span class="pre">Hello,</span> <span class="pre">world!</span></code> when run with
Python 3.2 or later.</p>
</div>
<div class="section" id="customizing-logrecord">
<span id="custom-logrecord"></span><h2>Customizing <code class="docutils literal"><span class="pre">LogRecord</span></code><a class="headerlink" href="#customizing-logrecord" title="Permalink to this headline">¶</a></h2>
<p>Every logging event is represented by a <a class="reference internal" href="../library/logging.html#logging.LogRecord" title="logging.LogRecord"><code class="xref py py-class docutils literal"><span class="pre">LogRecord</span></code></a> instance.
When an event is logged and not filtered out by a logger’s level, a
<a class="reference internal" href="../library/logging.html#logging.LogRecord" title="logging.LogRecord"><code class="xref py py-class docutils literal"><span class="pre">LogRecord</span></code></a> is created, populated with information about the event and
then passed to the handlers for that logger (and its ancestors, up to and
including the logger where further propagation up the hierarchy is disabled).
Before Python 3.2, there were only two places where this creation was done:</p>
<ul class="simple">
<li><a class="reference internal" href="../library/logging.html#logging.Logger.makeRecord" title="logging.Logger.makeRecord"><code class="xref py py-meth docutils literal"><span class="pre">Logger.makeRecord()</span></code></a>, which is called in the normal process of
logging an event. This invoked <a class="reference internal" href="../library/logging.html#logging.LogRecord" title="logging.LogRecord"><code class="xref py py-class docutils literal"><span class="pre">LogRecord</span></code></a> directly to create an
instance.</li>
<li><a class="reference internal" href="../library/logging.html#logging.makeLogRecord" title="logging.makeLogRecord"><code class="xref py py-func docutils literal"><span class="pre">makeLogRecord()</span></code></a>, which is called with a dictionary containing
attributes to be added to the LogRecord. This is typically invoked when a
suitable dictionary has been received over the network (e.g. in pickle form
via a <a class="reference internal" href="../library/logging.handlers.html#logging.handlers.SocketHandler" title="logging.handlers.SocketHandler"><code class="xref py py-class docutils literal"><span class="pre">SocketHandler</span></code></a>, or in JSON form via an
<a class="reference internal" href="../library/logging.handlers.html#logging.handlers.HTTPHandler" title="logging.handlers.HTTPHandler"><code class="xref py py-class docutils literal"><span class="pre">HTTPHandler</span></code></a>).</li>
</ul>
<p>This has usually meant that if you need to do anything special with a
<a class="reference internal" href="../library/logging.html#logging.LogRecord" title="logging.LogRecord"><code class="xref py py-class docutils literal"><span class="pre">LogRecord</span></code></a>, you’ve had to do one of the following.</p>
<ul class="simple">
<li>Create your own <a class="reference internal" href="../library/logging.html#logging.Logger" title="logging.Logger"><code class="xref py py-class docutils literal"><span class="pre">Logger</span></code></a> subclass, which overrides
<a class="reference internal" href="../library/logging.html#logging.Logger.makeRecord" title="logging.Logger.makeRecord"><code class="xref py py-meth docutils literal"><span class="pre">Logger.makeRecord()</span></code></a>, and set it using <a class="reference internal" href="../library/logging.html#logging.setLoggerClass" title="logging.setLoggerClass"><code class="xref py py-func docutils literal"><span class="pre">setLoggerClass()</span></code></a>
before any loggers that you care about are instantiated.</li>
<li>Add a <a class="reference internal" href="../library/logging.html#logging.Filter" title="logging.Filter"><code class="xref py py-class docutils literal"><span class="pre">Filter</span></code></a> to a logger or handler, which does the
necessary special manipulation you need when its
<a class="reference internal" href="../library/logging.html#logging.Filter.filter" title="logging.Filter.filter"><code class="xref py py-meth docutils literal"><span class="pre">filter()</span></code></a> method is called.</li>
</ul>
<p>The first approach would be a little unwieldy in the scenario where (say)
several different libraries wanted to do different things. Each would attempt
to set its own <a class="reference internal" href="../library/logging.html#logging.Logger" title="logging.Logger"><code class="xref py py-class docutils literal"><span class="pre">Logger</span></code></a> subclass, and the one which did this last would
win.</p>
<p>The second approach works reasonably well for many cases, but does not allow
you to e.g. use a specialized subclass of <a class="reference internal" href="../library/logging.html#logging.LogRecord" title="logging.LogRecord"><code class="xref py py-class docutils literal"><span class="pre">LogRecord</span></code></a>. Library
developers can set a suitable filter on their loggers, but they would have to
remember to do this every time they introduced a new logger (which they would
do simply by adding new packages or modules and doing</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">logger</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="n">__name__</span><span class="p">)</span>
</pre></div>
</div>
<p>at module level). It’s probably one too many things to think about. Developers
could also add the filter to a <a class="reference internal" href="../library/logging.handlers.html#logging.NullHandler" title="logging.NullHandler"><code class="xref py py-class docutils literal"><span class="pre">NullHandler</span></code></a> attached to their
top-level logger, but this would not be invoked if an application developer
attached a handler to a lower-level library logger — so output from that
handler would not reflect the intentions of the library developer.</p>
<p>In Python 3.2 and later, <a class="reference internal" href="../library/logging.html#logging.LogRecord" title="logging.LogRecord"><code class="xref py py-class docutils literal"><span class="pre">LogRecord</span></code></a> creation is done through a
factory, which you can specify. The factory is just a callable you can set with
<a class="reference internal" href="../library/logging.html#logging.setLogRecordFactory" title="logging.setLogRecordFactory"><code class="xref py py-func docutils literal"><span class="pre">setLogRecordFactory()</span></code></a>, and interrogate with
<a class="reference internal" href="../library/logging.html#logging.getLogRecordFactory" title="logging.getLogRecordFactory"><code class="xref py py-func docutils literal"><span class="pre">getLogRecordFactory()</span></code></a>. The factory is invoked with the same
signature as the <a class="reference internal" href="../library/logging.html#logging.LogRecord" title="logging.LogRecord"><code class="xref py py-class docutils literal"><span class="pre">LogRecord</span></code></a> constructor, as <a class="reference internal" href="../library/logging.html#logging.LogRecord" title="logging.LogRecord"><code class="xref py py-class docutils literal"><span class="pre">LogRecord</span></code></a>
is the default setting for the factory.</p>
<p>This approach allows a custom factory to control all aspects of LogRecord
creation. For example, you could return a subclass, or just add some additional
attributes to the record once created, using a pattern similar to this:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">old_factory</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogRecordFactory</span><span class="p">()</span>
<span class="k">def</span> <span class="nf">record_factory</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span>
<span class="n">record</span> <span class="o">=</span> <span class="n">old_factory</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">)</span>
<span class="n">record</span><span class="o">.</span><span class="n">custom_attribute</span> <span class="o">=</span> <span class="mh">0xdecafbad</span>
<span class="k">return</span> <span class="n">record</span>
<span class="n">logging</span><span class="o">.</span><span class="n">setLogRecordFactory</span><span class="p">(</span><span class="n">record_factory</span><span class="p">)</span>
</pre></div>
</div>
<p>This pattern allows different libraries to chain factories together, and as
long as they don’t overwrite each other’s attributes or unintentionally
overwrite the attributes provided as standard, there should be no surprises.
However, it should be borne in mind that each link in the chain adds run-time
overhead to all logging operations, and the technique should only be used when
the use of a <a class="reference internal" href="../library/logging.html#logging.Filter" title="logging.Filter"><code class="xref py py-class docutils literal"><span class="pre">Filter</span></code></a> does not provide the desired result.</p>
</div>
<div class="section" id="subclassing-queuehandler-a-zeromq-example">
<span id="zeromq-handlers"></span><h2>Subclassing QueueHandler - a ZeroMQ example<a class="headerlink" href="#subclassing-queuehandler-a-zeromq-example" title="Permalink to this headline">¶</a></h2>
<p>You can use a <code class="xref py py-class docutils literal"><span class="pre">QueueHandler</span></code> subclass to send messages to other kinds
of queues, for example a ZeroMQ ‘publish’ socket. In the example below,the
socket is created separately and passed to the handler (as its ‘queue’):</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">zmq</span> <span class="c1"># using pyzmq, the Python binding for ZeroMQ</span>
<span class="kn">import</span> <span class="nn">json</span> <span class="c1"># for serializing records portably</span>
<span class="n">ctx</span> <span class="o">=</span> <span class="n">zmq</span><span class="o">.</span><span class="n">Context</span><span class="p">()</span>
<span class="n">sock</span> <span class="o">=</span> <span class="n">zmq</span><span class="o">.</span><span class="n">Socket</span><span class="p">(</span><span class="n">ctx</span><span class="p">,</span> <span class="n">zmq</span><span class="o">.</span><span class="n">PUB</span><span class="p">)</span> <span class="c1"># or zmq.PUSH, or other suitable value</span>
<span class="n">sock</span><span class="o">.</span><span class="n">bind</span><span class="p">(</span><span class="s1">'tcp://*:5556'</span><span class="p">)</span> <span class="c1"># or wherever</span>
<span class="k">class</span> <span class="nc">ZeroMQSocketHandler</span><span class="p">(</span><span class="n">QueueHandler</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">enqueue</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">record</span><span class="p">):</span>
<span class="n">data</span> <span class="o">=</span> <span class="n">json</span><span class="o">.</span><span class="n">dumps</span><span class="p">(</span><span class="n">record</span><span class="o">.</span><span class="n">__dict__</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">queue</span><span class="o">.</span><span class="n">send</span><span class="p">(</span><span class="n">data</span><span class="p">)</span>
<span class="n">handler</span> <span class="o">=</span> <span class="n">ZeroMQSocketHandler</span><span class="p">(</span><span class="n">sock</span><span class="p">)</span>
</pre></div>
</div>
<p>Of course there are other ways of organizing this, for example passing in the
data needed by the handler to create the socket:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">ZeroMQSocketHandler</span><span class="p">(</span><span class="n">QueueHandler</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">uri</span><span class="p">,</span> <span class="n">socktype</span><span class="o">=</span><span class="n">zmq</span><span class="o">.</span><span class="n">PUB</span><span class="p">,</span> <span class="n">ctx</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">ctx</span> <span class="o">=</span> <span class="n">ctx</span> <span class="ow">or</span> <span class="n">zmq</span><span class="o">.</span><span class="n">Context</span><span class="p">()</span>
<span class="n">socket</span> <span class="o">=</span> <span class="n">zmq</span><span class="o">.</span><span class="n">Socket</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">ctx</span><span class="p">,</span> <span class="n">socktype</span><span class="p">)</span>
<span class="n">socket</span><span class="o">.</span><span class="n">bind</span><span class="p">(</span><span class="n">uri</span><span class="p">)</span>
<span class="n">QueueHandler</span><span class="o">.</span><span class="n">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">socket</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">enqueue</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">record</span><span class="p">):</span>
<span class="n">data</span> <span class="o">=</span> <span class="n">json</span><span class="o">.</span><span class="n">dumps</span><span class="p">(</span><span class="n">record</span><span class="o">.</span><span class="n">__dict__</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">queue</span><span class="o">.</span><span class="n">send</span><span class="p">(</span><span class="n">data</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">close</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">queue</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
</pre></div>
</div>
</div>
<div class="section" id="subclassing-queuelistener-a-zeromq-example">
<h2>Subclassing QueueListener - a ZeroMQ example<a class="headerlink" href="#subclassing-queuelistener-a-zeromq-example" title="Permalink to this headline">¶</a></h2>
<p>You can also subclass <code class="xref py py-class docutils literal"><span class="pre">QueueListener</span></code> to get messages from other kinds
of queues, for example a ZeroMQ ‘subscribe’ socket. Here’s an example:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">ZeroMQSocketListener</span><span class="p">(</span><span class="n">QueueListener</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">uri</span><span class="p">,</span> <span class="o">*</span><span class="n">handlers</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">ctx</span> <span class="o">=</span> <span class="n">kwargs</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="s1">'ctx'</span><span class="p">)</span> <span class="ow">or</span> <span class="n">zmq</span><span class="o">.</span><span class="n">Context</span><span class="p">()</span>
<span class="n">socket</span> <span class="o">=</span> <span class="n">zmq</span><span class="o">.</span><span class="n">Socket</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">ctx</span><span class="p">,</span> <span class="n">zmq</span><span class="o">.</span><span class="n">SUB</span><span class="p">)</span>
<span class="n">socket</span><span class="o">.</span><span class="n">setsockopt</span><span class="p">(</span><span class="n">zmq</span><span class="o">.</span><span class="n">SUBSCRIBE</span><span class="p">,</span> <span class="s1">''</span><span class="p">)</span> <span class="c1"># subscribe to everything</span>
<span class="n">socket</span><span class="o">.</span><span class="n">connect</span><span class="p">(</span><span class="n">uri</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">dequeue</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="n">msg</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">queue</span><span class="o">.</span><span class="n">recv</span><span class="p">()</span>
<span class="k">return</span> <span class="n">logging</span><span class="o">.</span><span class="n">makeLogRecord</span><span class="p">(</span><span class="n">json</span><span class="o">.</span><span class="n">loads</span><span class="p">(</span><span class="n">msg</span><span class="p">))</span>
</pre></div>
</div>
<div class="admonition seealso">
<p class="first admonition-title">See also</p>
<dl class="docutils">
<dt>Module <a class="reference internal" href="../library/logging.html#module-logging" title="logging: Flexible event logging system for applications."><code class="xref py py-mod docutils literal"><span class="pre">logging</span></code></a></dt>
<dd>API reference for the logging module.</dd>
<dt>Module <a class="reference internal" href="../library/logging.config.html#module-logging.config" title="logging.config: Configuration of the logging module."><code class="xref py py-mod docutils literal"><span class="pre">logging.config</span></code></a></dt>
<dd>Configuration API for the logging module.</dd>
<dt>Module <a class="reference internal" href="../library/logging.handlers.html#module-logging.handlers" title="logging.handlers: Handlers for the logging module."><code class="xref py py-mod docutils literal"><span class="pre">logging.handlers</span></code></a></dt>
<dd>Useful handlers included with the logging module.</dd>
</dl>
<p><a class="reference internal" href="logging.html#logging-basic-tutorial"><span>A basic logging tutorial</span></a></p>
<p class="last"><a class="reference internal" href="logging.html#logging-advanced-tutorial"><span>A more advanced logging tutorial</span></a></p>
</div>
</div>
<div class="section" id="an-example-dictionary-based-configuration">
<h2>An example dictionary-based configuration<a class="headerlink" href="#an-example-dictionary-based-configuration" title="Permalink to this headline">¶</a></h2>
<p>Below is an example of a logging configuration dictionary - it’s taken from
the <a class="reference external" href="https://docs.djangoproject.com/en/1.9/topics/logging/#configuring-logging">documentation on the Django project</a>.
This dictionary is passed to <a class="reference internal" href="../library/logging.config.html#logging.config.dictConfig" title="logging.config.dictConfig"><code class="xref py py-func docutils literal"><span class="pre">dictConfig()</span></code></a> to put the configuration into effect:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">LOGGING</span> <span class="o">=</span> <span class="p">{</span>
<span class="s1">'version'</span><span class="p">:</span> <span class="mi">1</span><span class="p">,</span>
<span class="s1">'disable_existing_loggers'</span><span class="p">:</span> <span class="kc">True</span><span class="p">,</span>
<span class="s1">'formatters'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'verbose'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'format'</span><span class="p">:</span> <span class="s1">'</span><span class="si">%(levelname)s</span><span class="s1"> </span><span class="si">%(asctime)s</span><span class="s1"> </span><span class="si">%(module)s</span><span class="s1"> </span><span class="si">%(process)d</span><span class="s1"> </span><span class="si">%(thread)d</span><span class="s1"> </span><span class="si">%(message)s</span><span class="s1">'</span>
<span class="p">},</span>
<span class="s1">'simple'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'format'</span><span class="p">:</span> <span class="s1">'</span><span class="si">%(levelname)s</span><span class="s1"> </span><span class="si">%(message)s</span><span class="s1">'</span>
<span class="p">},</span>
<span class="p">},</span>
<span class="s1">'filters'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'special'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'()'</span><span class="p">:</span> <span class="s1">'project.logging.SpecialFilter'</span><span class="p">,</span>
<span class="s1">'foo'</span><span class="p">:</span> <span class="s1">'bar'</span><span class="p">,</span>
<span class="p">}</span>
<span class="p">},</span>
<span class="s1">'handlers'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'null'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'level'</span><span class="p">:</span><span class="s1">'DEBUG'</span><span class="p">,</span>
<span class="s1">'class'</span><span class="p">:</span><span class="s1">'django.utils.log.NullHandler'</span><span class="p">,</span>
<span class="p">},</span>
<span class="s1">'console'</span><span class="p">:{</span>
<span class="s1">'level'</span><span class="p">:</span><span class="s1">'DEBUG'</span><span class="p">,</span>
<span class="s1">'class'</span><span class="p">:</span><span class="s1">'logging.StreamHandler'</span><span class="p">,</span>
<span class="s1">'formatter'</span><span class="p">:</span> <span class="s1">'simple'</span>
<span class="p">},</span>
<span class="s1">'mail_admins'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'level'</span><span class="p">:</span> <span class="s1">'ERROR'</span><span class="p">,</span>
<span class="s1">'class'</span><span class="p">:</span> <span class="s1">'django.utils.log.AdminEmailHandler'</span><span class="p">,</span>
<span class="s1">'filters'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'special'</span><span class="p">]</span>
<span class="p">}</span>
<span class="p">},</span>
<span class="s1">'loggers'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'django'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'handlers'</span><span class="p">:[</span><span class="s1">'null'</span><span class="p">],</span>
<span class="s1">'propagate'</span><span class="p">:</span> <span class="kc">True</span><span class="p">,</span>
<span class="s1">'level'</span><span class="p">:</span><span class="s1">'INFO'</span><span class="p">,</span>
<span class="p">},</span>
<span class="s1">'django.request'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'handlers'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'mail_admins'</span><span class="p">],</span>
<span class="s1">'level'</span><span class="p">:</span> <span class="s1">'ERROR'</span><span class="p">,</span>
<span class="s1">'propagate'</span><span class="p">:</span> <span class="kc">False</span><span class="p">,</span>
<span class="p">},</span>
<span class="s1">'myproject.custom'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'handlers'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'console'</span><span class="p">,</span> <span class="s1">'mail_admins'</span><span class="p">],</span>
<span class="s1">'level'</span><span class="p">:</span> <span class="s1">'INFO'</span><span class="p">,</span>
<span class="s1">'filters'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'special'</span><span class="p">]</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
</div>
<p>For more information about this configuration, you can see the <a class="reference external" href="https://docs.djangoproject.com/en/1.9/topics/logging/#configuring-logging">relevant
section</a>
of the Django documentation.</p>
</div>
<div class="section" id="using-a-rotator-and-namer-to-customize-log-rotation-processing">
<span id="cookbook-rotator-namer"></span><h2>Using a rotator and namer to customize log rotation processing<a class="headerlink" href="#using-a-rotator-and-namer-to-customize-log-rotation-processing" title="Permalink to this headline">¶</a></h2>
<p>An example of how you can define a namer and rotator is given in the following
snippet, which shows zlib-based compression of the log file:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">namer</span><span class="p">(</span><span class="n">name</span><span class="p">):</span>
<span class="k">return</span> <span class="n">name</span> <span class="o">+</span> <span class="s2">".gz"</span>
<span class="k">def</span> <span class="nf">rotator</span><span class="p">(</span><span class="n">source</span><span class="p">,</span> <span class="n">dest</span><span class="p">):</span>
<span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="n">source</span><span class="p">,</span> <span class="s2">"rb"</span><span class="p">)</span> <span class="k">as</span> <span class="n">sf</span><span class="p">:</span>
<span class="n">data</span> <span class="o">=</span> <span class="n">sf</span><span class="o">.</span><span class="n">read</span><span class="p">()</span>
<span class="n">compressed</span> <span class="o">=</span> <span class="n">zlib</span><span class="o">.</span><span class="n">compress</span><span class="p">(</span><span class="n">data</span><span class="p">,</span> <span class="mi">9</span><span class="p">)</span>
<span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="n">dest</span><span class="p">,</span> <span class="s2">"wb"</span><span class="p">)</span> <span class="k">as</span> <span class="n">df</span><span class="p">:</span>
<span class="n">df</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="n">compressed</span><span class="p">)</span>
<span class="n">os</span><span class="o">.</span><span class="n">remove</span><span class="p">(</span><span class="n">source</span><span class="p">)</span>
<span class="n">rh</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">handlers</span><span class="o">.</span><span class="n">RotatingFileHandler</span><span class="p">(</span><span class="o">...</span><span class="p">)</span>
<span class="n">rh</span><span class="o">.</span><span class="n">rotator</span> <span class="o">=</span> <span class="n">rotator</span>
<span class="n">rh</span><span class="o">.</span><span class="n">namer</span> <span class="o">=</span> <span class="n">namer</span>
</pre></div>
</div>
<p>These are not “true” .gz files, as they are bare compressed data, with no
“container” such as you’d find in an actual gzip file. This snippet is just
for illustration purposes.</p>
</div>
<div class="section" id="a-more-elaborate-multiprocessing-example">
<h2>A more elaborate multiprocessing example<a class="headerlink" href="#a-more-elaborate-multiprocessing-example" title="Permalink to this headline">¶</a></h2>
<p>The following working example shows how logging can be used with multiprocessing
using configuration files. The configurations are fairly simple, but serve to
illustrate how more complex ones could be implemented in a real multiprocessing
scenario.</p>
<p>In the example, the main process spawns a listener process and some worker
processes. Each of the main process, the listener and the workers have three
separate configurations (the workers all share the same configuration). We can
see logging in the main process, how the workers log to a QueueHandler and how
the listener implements a QueueListener and a more complex logging
configuration, and arranges to dispatch events received via the queue to the
handlers specified in the configuration. Note that these configurations are
purely illustrative, but you should be able to adapt this example to your own
scenario.</p>
<p>Here’s the script - the docstrings and the comments hopefully explain how it
works:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">logging</span>
<span class="kn">import</span> <span class="nn">logging.config</span>
<span class="kn">import</span> <span class="nn">logging.handlers</span>
<span class="kn">from</span> <span class="nn">multiprocessing</span> <span class="k">import</span> <span class="n">Process</span><span class="p">,</span> <span class="n">Queue</span><span class="p">,</span> <span class="n">Event</span><span class="p">,</span> <span class="n">current_process</span>
<span class="kn">import</span> <span class="nn">os</span>
<span class="kn">import</span> <span class="nn">random</span>
<span class="kn">import</span> <span class="nn">time</span>
<span class="k">class</span> <span class="nc">MyHandler</span><span class="p">:</span>
<span class="sd">"""</span>
<span class="sd"> A simple handler for logging events. It runs in the listener process and</span>
<span class="sd"> dispatches events to loggers based on the name in the received record,</span>
<span class="sd"> which then get dispatched, by the logging system, to the handlers</span>
<span class="sd"> configured for those loggers.</span>
<span class="sd"> """</span>
<span class="k">def</span> <span class="nf">handle</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">record</span><span class="p">):</span>
<span class="n">logger</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="n">record</span><span class="o">.</span><span class="n">name</span><span class="p">)</span>
<span class="c1"># The process name is transformed just to show that it's the listener</span>
<span class="c1"># doing the logging to files and console</span>
<span class="n">record</span><span class="o">.</span><span class="n">processName</span> <span class="o">=</span> <span class="s1">'</span><span class="si">%s</span><span class="s1"> (for </span><span class="si">%s</span><span class="s1">)'</span> <span class="o">%</span> <span class="p">(</span><span class="n">current_process</span><span class="p">()</span><span class="o">.</span><span class="n">name</span><span class="p">,</span> <span class="n">record</span><span class="o">.</span><span class="n">processName</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">handle</span><span class="p">(</span><span class="n">record</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">listener_process</span><span class="p">(</span><span class="n">q</span><span class="p">,</span> <span class="n">stop_event</span><span class="p">,</span> <span class="n">config</span><span class="p">):</span>
<span class="sd">"""</span>
<span class="sd"> This could be done in the main process, but is just done in a separate</span>
<span class="sd"> process for illustrative purposes.</span>
<span class="sd"> This initialises logging according to the specified configuration,</span>
<span class="sd"> starts the listener and waits for the main process to signal completion</span>
<span class="sd"> via the event. The listener is then stopped, and the process exits.</span>
<span class="sd"> """</span>
<span class="n">logging</span><span class="o">.</span><span class="n">config</span><span class="o">.</span><span class="n">dictConfig</span><span class="p">(</span><span class="n">config</span><span class="p">)</span>
<span class="n">listener</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">handlers</span><span class="o">.</span><span class="n">QueueListener</span><span class="p">(</span><span class="n">q</span><span class="p">,</span> <span class="n">MyHandler</span><span class="p">())</span>
<span class="n">listener</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="k">if</span> <span class="n">os</span><span class="o">.</span><span class="n">name</span> <span class="o">==</span> <span class="s1">'posix'</span><span class="p">:</span>
<span class="c1"># On POSIX, the setup logger will have been configured in the</span>
<span class="c1"># parent process, but should have been disabled following the</span>
<span class="c1"># dictConfig call.</span>
<span class="c1"># On Windows, since fork isn't used, the setup logger won't</span>
<span class="c1"># exist in the child, so it would be created and the message</span>
<span class="c1"># would appear - hence the "if posix" clause.</span>
<span class="n">logger</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="s1">'setup'</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">critical</span><span class="p">(</span><span class="s1">'Should not appear, because of disabled logger ...'</span><span class="p">)</span>
<span class="n">stop_event</span><span class="o">.</span><span class="n">wait</span><span class="p">()</span>
<span class="n">listener</span><span class="o">.</span><span class="n">stop</span><span class="p">()</span>
<span class="k">def</span> <span class="nf">worker_process</span><span class="p">(</span><span class="n">config</span><span class="p">):</span>
<span class="sd">"""</span>
<span class="sd"> A number of these are spawned for the purpose of illustration. In</span>
<span class="sd"> practice, they could be a heterogeneous bunch of processes rather than</span>
<span class="sd"> ones which are identical to each other.</span>
<span class="sd"> This initialises logging according to the specified configuration,</span>
<span class="sd"> and logs a hundred messages with random levels to randomly selected</span>
<span class="sd"> loggers.</span>
<span class="sd"> A small sleep is added to allow other processes a chance to run. This</span>
<span class="sd"> is not strictly needed, but it mixes the output from the different</span>
<span class="sd"> processes a bit more than if it's left out.</span>
<span class="sd"> """</span>
<span class="n">logging</span><span class="o">.</span><span class="n">config</span><span class="o">.</span><span class="n">dictConfig</span><span class="p">(</span><span class="n">config</span><span class="p">)</span>
<span class="n">levels</span> <span class="o">=</span> <span class="p">[</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">,</span> <span class="n">logging</span><span class="o">.</span><span class="n">INFO</span><span class="p">,</span> <span class="n">logging</span><span class="o">.</span><span class="n">WARNING</span><span class="p">,</span> <span class="n">logging</span><span class="o">.</span><span class="n">ERROR</span><span class="p">,</span>
<span class="n">logging</span><span class="o">.</span><span class="n">CRITICAL</span><span class="p">]</span>
<span class="n">loggers</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'foo'</span><span class="p">,</span> <span class="s1">'foo.bar'</span><span class="p">,</span> <span class="s1">'foo.bar.baz'</span><span class="p">,</span>
<span class="s1">'spam'</span><span class="p">,</span> <span class="s1">'spam.ham'</span><span class="p">,</span> <span class="s1">'spam.ham.eggs'</span><span class="p">]</span>
<span class="k">if</span> <span class="n">os</span><span class="o">.</span><span class="n">name</span> <span class="o">==</span> <span class="s1">'posix'</span><span class="p">:</span>
<span class="c1"># On POSIX, the setup logger will have been configured in the</span>
<span class="c1"># parent process, but should have been disabled following the</span>
<span class="c1"># dictConfig call.</span>
<span class="c1"># On Windows, since fork isn't used, the setup logger won't</span>
<span class="c1"># exist in the child, so it would be created and the message</span>
<span class="c1"># would appear - hence the "if posix" clause.</span>
<span class="n">logger</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="s1">'setup'</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">critical</span><span class="p">(</span><span class="s1">'Should not appear, because of disabled logger ...'</span><span class="p">)</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">100</span><span class="p">):</span>
<span class="n">lvl</span> <span class="o">=</span> <span class="n">random</span><span class="o">.</span><span class="n">choice</span><span class="p">(</span><span class="n">levels</span><span class="p">)</span>
<span class="n">logger</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="n">random</span><span class="o">.</span><span class="n">choice</span><span class="p">(</span><span class="n">loggers</span><span class="p">))</span>
<span class="n">logger</span><span class="o">.</span><span class="n">log</span><span class="p">(</span><span class="n">lvl</span><span class="p">,</span> <span class="s1">'Message no. </span><span class="si">%d</span><span class="s1">'</span><span class="p">,</span> <span class="n">i</span><span class="p">)</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mf">0.01</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">main</span><span class="p">():</span>
<span class="n">q</span> <span class="o">=</span> <span class="n">Queue</span><span class="p">()</span>
<span class="c1"># The main process gets a simple configuration which prints to the console.</span>
<span class="n">config_initial</span> <span class="o">=</span> <span class="p">{</span>
<span class="s1">'version'</span><span class="p">:</span> <span class="mi">1</span><span class="p">,</span>
<span class="s1">'formatters'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'detailed'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'class'</span><span class="p">:</span> <span class="s1">'logging.Formatter'</span><span class="p">,</span>
<span class="s1">'format'</span><span class="p">:</span> <span class="s1">'</span><span class="si">%(asctime)s</span><span class="s1"> </span><span class="si">%(name)-15s</span><span class="s1"> </span><span class="si">%(levelname)-8s</span><span class="s1"> </span><span class="si">%(processName)-10s</span><span class="s1"> </span><span class="si">%(message)s</span><span class="s1">'</span>
<span class="p">}</span>
<span class="p">},</span>
<span class="s1">'handlers'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'console'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'class'</span><span class="p">:</span> <span class="s1">'logging.StreamHandler'</span><span class="p">,</span>
<span class="s1">'level'</span><span class="p">:</span> <span class="s1">'INFO'</span><span class="p">,</span>
<span class="p">},</span>
<span class="p">},</span>
<span class="s1">'root'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'level'</span><span class="p">:</span> <span class="s1">'DEBUG'</span><span class="p">,</span>
<span class="s1">'handlers'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'console'</span><span class="p">]</span>
<span class="p">},</span>
<span class="p">}</span>
<span class="c1"># The worker process configuration is just a QueueHandler attached to the</span>
<span class="c1"># root logger, which allows all messages to be sent to the queue.</span>
<span class="c1"># We disable existing loggers to disable the "setup" logger used in the</span>
<span class="c1"># parent process. This is needed on POSIX because the logger will</span>
<span class="c1"># be there in the child following a fork().</span>
<span class="n">config_worker</span> <span class="o">=</span> <span class="p">{</span>
<span class="s1">'version'</span><span class="p">:</span> <span class="mi">1</span><span class="p">,</span>
<span class="s1">'disable_existing_loggers'</span><span class="p">:</span> <span class="kc">True</span><span class="p">,</span>
<span class="s1">'handlers'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'queue'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'class'</span><span class="p">:</span> <span class="s1">'logging.handlers.QueueHandler'</span><span class="p">,</span>
<span class="s1">'queue'</span><span class="p">:</span> <span class="n">q</span><span class="p">,</span>
<span class="p">},</span>
<span class="p">},</span>
<span class="s1">'root'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'level'</span><span class="p">:</span> <span class="s1">'DEBUG'</span><span class="p">,</span>
<span class="s1">'handlers'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'queue'</span><span class="p">]</span>
<span class="p">},</span>
<span class="p">}</span>
<span class="c1"># The listener process configuration shows that the full flexibility of</span>
<span class="c1"># logging configuration is available to dispatch events to handlers however</span>
<span class="c1"># you want.</span>
<span class="c1"># We disable existing loggers to disable the "setup" logger used in the</span>
<span class="c1"># parent process. This is needed on POSIX because the logger will</span>
<span class="c1"># be there in the child following a fork().</span>
<span class="n">config_listener</span> <span class="o">=</span> <span class="p">{</span>
<span class="s1">'version'</span><span class="p">:</span> <span class="mi">1</span><span class="p">,</span>
<span class="s1">'disable_existing_loggers'</span><span class="p">:</span> <span class="kc">True</span><span class="p">,</span>
<span class="s1">'formatters'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'detailed'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'class'</span><span class="p">:</span> <span class="s1">'logging.Formatter'</span><span class="p">,</span>
<span class="s1">'format'</span><span class="p">:</span> <span class="s1">'</span><span class="si">%(asctime)s</span><span class="s1"> </span><span class="si">%(name)-15s</span><span class="s1"> </span><span class="si">%(levelname)-8s</span><span class="s1"> </span><span class="si">%(processName)-10s</span><span class="s1"> </span><span class="si">%(message)s</span><span class="s1">'</span>
<span class="p">},</span>
<span class="s1">'simple'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'class'</span><span class="p">:</span> <span class="s1">'logging.Formatter'</span><span class="p">,</span>
<span class="s1">'format'</span><span class="p">:</span> <span class="s1">'</span><span class="si">%(name)-15s</span><span class="s1"> </span><span class="si">%(levelname)-8s</span><span class="s1"> </span><span class="si">%(processName)-10s</span><span class="s1"> </span><span class="si">%(message)s</span><span class="s1">'</span>
<span class="p">}</span>
<span class="p">},</span>
<span class="s1">'handlers'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'console'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'class'</span><span class="p">:</span> <span class="s1">'logging.StreamHandler'</span><span class="p">,</span>
<span class="s1">'level'</span><span class="p">:</span> <span class="s1">'INFO'</span><span class="p">,</span>
<span class="s1">'formatter'</span><span class="p">:</span> <span class="s1">'simple'</span><span class="p">,</span>
<span class="p">},</span>
<span class="s1">'file'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'class'</span><span class="p">:</span> <span class="s1">'logging.FileHandler'</span><span class="p">,</span>
<span class="s1">'filename'</span><span class="p">:</span> <span class="s1">'mplog.log'</span><span class="p">,</span>
<span class="s1">'mode'</span><span class="p">:</span> <span class="s1">'w'</span><span class="p">,</span>
<span class="s1">'formatter'</span><span class="p">:</span> <span class="s1">'detailed'</span><span class="p">,</span>
<span class="p">},</span>
<span class="s1">'foofile'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'class'</span><span class="p">:</span> <span class="s1">'logging.FileHandler'</span><span class="p">,</span>
<span class="s1">'filename'</span><span class="p">:</span> <span class="s1">'mplog-foo.log'</span><span class="p">,</span>
<span class="s1">'mode'</span><span class="p">:</span> <span class="s1">'w'</span><span class="p">,</span>
<span class="s1">'formatter'</span><span class="p">:</span> <span class="s1">'detailed'</span><span class="p">,</span>
<span class="p">},</span>
<span class="s1">'errors'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'class'</span><span class="p">:</span> <span class="s1">'logging.FileHandler'</span><span class="p">,</span>
<span class="s1">'filename'</span><span class="p">:</span> <span class="s1">'mplog-errors.log'</span><span class="p">,</span>
<span class="s1">'mode'</span><span class="p">:</span> <span class="s1">'w'</span><span class="p">,</span>
<span class="s1">'level'</span><span class="p">:</span> <span class="s1">'ERROR'</span><span class="p">,</span>
<span class="s1">'formatter'</span><span class="p">:</span> <span class="s1">'detailed'</span><span class="p">,</span>
<span class="p">},</span>
<span class="p">},</span>
<span class="s1">'loggers'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'foo'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'handlers'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'foofile'</span><span class="p">]</span>
<span class="p">}</span>
<span class="p">},</span>
<span class="s1">'root'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'level'</span><span class="p">:</span> <span class="s1">'DEBUG'</span><span class="p">,</span>
<span class="s1">'handlers'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'console'</span><span class="p">,</span> <span class="s1">'file'</span><span class="p">,</span> <span class="s1">'errors'</span><span class="p">]</span>
<span class="p">},</span>
<span class="p">}</span>
<span class="c1"># Log some initial events, just to show that logging in the parent works</span>
<span class="c1"># normally.</span>
<span class="n">logging</span><span class="o">.</span><span class="n">config</span><span class="o">.</span><span class="n">dictConfig</span><span class="p">(</span><span class="n">config_initial</span><span class="p">)</span>
<span class="n">logger</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="s1">'setup'</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'About to create workers ...'</span><span class="p">)</span>
<span class="n">workers</span> <span class="o">=</span> <span class="p">[]</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">5</span><span class="p">):</span>
<span class="n">wp</span> <span class="o">=</span> <span class="n">Process</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">worker_process</span><span class="p">,</span> <span class="n">name</span><span class="o">=</span><span class="s1">'worker </span><span class="si">%d</span><span class="s1">'</span> <span class="o">%</span> <span class="p">(</span><span class="n">i</span> <span class="o">+</span> <span class="mi">1</span><span class="p">),</span>
<span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">config_worker</span><span class="p">,))</span>
<span class="n">workers</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">wp</span><span class="p">)</span>
<span class="n">wp</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="n">logger</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'Started worker: </span><span class="si">%s</span><span class="s1">'</span><span class="p">,</span> <span class="n">wp</span><span class="o">.</span><span class="n">name</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'About to create listener ...'</span><span class="p">)</span>
<span class="n">stop_event</span> <span class="o">=</span> <span class="n">Event</span><span class="p">()</span>
<span class="n">lp</span> <span class="o">=</span> <span class="n">Process</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">listener_process</span><span class="p">,</span> <span class="n">name</span><span class="o">=</span><span class="s1">'listener'</span><span class="p">,</span>
<span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">q</span><span class="p">,</span> <span class="n">stop_event</span><span class="p">,</span> <span class="n">config_listener</span><span class="p">))</span>
<span class="n">lp</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="n">logger</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'Started listener'</span><span class="p">)</span>
<span class="c1"># We now hang around for the workers to finish their work.</span>
<span class="k">for</span> <span class="n">wp</span> <span class="ow">in</span> <span class="n">workers</span><span class="p">:</span>
<span class="n">wp</span><span class="o">.</span><span class="n">join</span><span class="p">()</span>
<span class="c1"># Workers all done, listening can now stop.</span>
<span class="c1"># Logging in the parent still works normally.</span>
<span class="n">logger</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'Telling listener to stop ...'</span><span class="p">)</span>
<span class="n">stop_event</span><span class="o">.</span><span class="n">set</span><span class="p">()</span>
<span class="n">lp</span><span class="o">.</span><span class="n">join</span><span class="p">()</span>
<span class="n">logger</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'All done.'</span><span class="p">)</span>
<span class="k">if</span> <span class="n">__name__</span> <span class="o">==</span> <span class="s1">'__main__'</span><span class="p">:</span>
<span class="n">main</span><span class="p">()</span>
</pre></div>
</div>
</div>
<div class="section" id="inserting-a-bom-into-messages-sent-to-a-sysloghandler">
<h2>Inserting a BOM into messages sent to a SysLogHandler<a class="headerlink" href="#inserting-a-bom-into-messages-sent-to-a-sysloghandler" title="Permalink to this headline">¶</a></h2>
<p><a class="reference external" href="https://tools.ietf.org/html/rfc5424">RFC 5424</a> requires that a
Unicode message be sent to a syslog daemon as a set of bytes which have the
following structure: an optional pure-ASCII component, followed by a UTF-8 Byte
Order Mark (BOM), followed by Unicode encoded using UTF-8. (See the <a class="reference external" href="https://tools.ietf.org/html/rfc5424#section-6">relevant
section of the specification</a>.)</p>
<p>In Python 3.1, code was added to
<a class="reference internal" href="../library/logging.handlers.html#logging.handlers.SysLogHandler" title="logging.handlers.SysLogHandler"><code class="xref py py-class docutils literal"><span class="pre">SysLogHandler</span></code></a> to insert a BOM into the message, but
unfortunately, it was implemented incorrectly, with the BOM appearing at the
beginning of the message and hence not allowing any pure-ASCII component to
appear before it.</p>
<p>As this behaviour is broken, the incorrect BOM insertion code is being removed
from Python 3.2.4 and later. However, it is not being replaced, and if you
want to produce RFC 5424-compliant messages which include a BOM, an optional
pure-ASCII sequence before it and arbitrary Unicode after it, encoded using
UTF-8, then you need to do the following:</p>
<ol class="arabic">
<li><p class="first">Attach a <a class="reference internal" href="../library/logging.html#logging.Formatter" title="logging.Formatter"><code class="xref py py-class docutils literal"><span class="pre">Formatter</span></code></a> instance to your
<a class="reference internal" href="../library/logging.handlers.html#logging.handlers.SysLogHandler" title="logging.handlers.SysLogHandler"><code class="xref py py-class docutils literal"><span class="pre">SysLogHandler</span></code></a> instance, with a format string
such as:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="s1">'ASCII section</span><span class="se">\ufeff</span><span class="s1">Unicode section'</span>
</pre></div>
</div>
<p>The Unicode code point U+FEFF, when encoded using UTF-8, will be
encoded as a UTF-8 BOM – the byte-string <code class="docutils literal"><span class="pre">b'\xef\xbb\xbf'</span></code>.</p>
</li>
<li><p class="first">Replace the ASCII section with whatever placeholders you like, but make sure
that the data that appears in there after substitution is always ASCII (that
way, it will remain unchanged after UTF-8 encoding).</p>
</li>
<li><p class="first">Replace the Unicode section with whatever placeholders you like; if the data
which appears there after substitution contains characters outside the ASCII
range, that’s fine – it will be encoded using UTF-8.</p>
</li>
</ol>
<p>The formatted message <em>will</em> be encoded using UTF-8 encoding by
<code class="docutils literal"><span class="pre">SysLogHandler</span></code>. If you follow the above rules, you should be able to produce
RFC 5424-compliant messages. If you don’t, logging may not complain, but your
messages will not be RFC 5424-compliant, and your syslog daemon may complain.</p>
</div>
<div class="section" id="implementing-structured-logging">
<h2>Implementing structured logging<a class="headerlink" href="#implementing-structured-logging" title="Permalink to this headline">¶</a></h2>
<p>Although most logging messages are intended for reading by humans, and thus not
readily machine-parseable, there might be cirumstances where you want to output
messages in a structured format which <em>is</em> capable of being parsed by a program
(without needing complex regular expressions to parse the log message). This is
straightforward to achieve using the logging package. There are a number of
ways in which this could be achieved, but the following is a simple approach
which uses JSON to serialise the event in a machine-parseable manner:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">json</span>
<span class="kn">import</span> <span class="nn">logging</span>
<span class="k">class</span> <span class="nc">StructuredMessage</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">message</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">message</span> <span class="o">=</span> <span class="n">message</span>
<span class="bp">self</span><span class="o">.</span><span class="n">kwargs</span> <span class="o">=</span> <span class="n">kwargs</span>
<span class="k">def</span> <span class="nf">__str__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</span> <span class="s1">'</span><span class="si">%s</span><span class="s1"> >>> </span><span class="si">%s</span><span class="s1">'</span> <span class="o">%</span> <span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">message</span><span class="p">,</span> <span class="n">json</span><span class="o">.</span><span class="n">dumps</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">kwargs</span><span class="p">))</span>
<span class="n">_</span> <span class="o">=</span> <span class="n">StructuredMessage</span> <span class="c1"># optional, to improve readability</span>
<span class="n">logging</span><span class="o">.</span><span class="n">basicConfig</span><span class="p">(</span><span class="n">level</span><span class="o">=</span><span class="n">logging</span><span class="o">.</span><span class="n">INFO</span><span class="p">,</span> <span class="nb">format</span><span class="o">=</span><span class="s1">'</span><span class="si">%(message)s</span><span class="s1">'</span><span class="p">)</span>
<span class="n">logging</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="n">_</span><span class="p">(</span><span class="s1">'message 1'</span><span class="p">,</span> <span class="n">foo</span><span class="o">=</span><span class="s1">'bar'</span><span class="p">,</span> <span class="n">bar</span><span class="o">=</span><span class="s1">'baz'</span><span class="p">,</span> <span class="n">num</span><span class="o">=</span><span class="mi">123</span><span class="p">,</span> <span class="n">fnum</span><span class="o">=</span><span class="mf">123.456</span><span class="p">))</span>
</pre></div>
</div>
<p>If the above script is run, it prints:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">message</span> <span class="mi">1</span> <span class="o">>>></span> <span class="p">{</span><span class="s2">"fnum"</span><span class="p">:</span> <span class="mf">123.456</span><span class="p">,</span> <span class="s2">"num"</span><span class="p">:</span> <span class="mi">123</span><span class="p">,</span> <span class="s2">"bar"</span><span class="p">:</span> <span class="s2">"baz"</span><span class="p">,</span> <span class="s2">"foo"</span><span class="p">:</span> <span class="s2">"bar"</span><span class="p">}</span>
</pre></div>
</div>
<p>Note that the order of items might be different according to the version of
Python used.</p>
<p>If you need more specialised processing, you can use a custom JSON encoder,
as in the following complete example:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">__future__</span> <span class="k">import</span> <span class="n">unicode_literals</span>
<span class="kn">import</span> <span class="nn">json</span>
<span class="kn">import</span> <span class="nn">logging</span>
<span class="c1"># This next bit is to ensure the script runs unchanged on 2.x and 3.x</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">unicode</span>
<span class="k">except</span> <span class="ne">NameError</span><span class="p">:</span>
<span class="n">unicode</span> <span class="o">=</span> <span class="nb">str</span>
<span class="k">class</span> <span class="nc">Encoder</span><span class="p">(</span><span class="n">json</span><span class="o">.</span><span class="n">JSONEncoder</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">default</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">o</span><span class="p">):</span>
<span class="k">if</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">o</span><span class="p">,</span> <span class="nb">set</span><span class="p">):</span>
<span class="k">return</span> <span class="nb">tuple</span><span class="p">(</span><span class="n">o</span><span class="p">)</span>
<span class="k">elif</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">o</span><span class="p">,</span> <span class="n">unicode</span><span class="p">):</span>
<span class="k">return</span> <span class="n">o</span><span class="o">.</span><span class="n">encode</span><span class="p">(</span><span class="s1">'unicode_escape'</span><span class="p">)</span><span class="o">.</span><span class="n">decode</span><span class="p">(</span><span class="s1">'ascii'</span><span class="p">)</span>
<span class="k">return</span> <span class="nb">super</span><span class="p">(</span><span class="n">Encoder</span><span class="p">,</span> <span class="bp">self</span><span class="p">)</span><span class="o">.</span><span class="n">default</span><span class="p">(</span><span class="n">o</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">StructuredMessage</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">message</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">message</span> <span class="o">=</span> <span class="n">message</span>
<span class="bp">self</span><span class="o">.</span><span class="n">kwargs</span> <span class="o">=</span> <span class="n">kwargs</span>
<span class="k">def</span> <span class="nf">__str__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="n">s</span> <span class="o">=</span> <span class="n">Encoder</span><span class="p">()</span><span class="o">.</span><span class="n">encode</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">kwargs</span><span class="p">)</span>
<span class="k">return</span> <span class="s1">'</span><span class="si">%s</span><span class="s1"> >>> </span><span class="si">%s</span><span class="s1">'</span> <span class="o">%</span> <span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">message</span><span class="p">,</span> <span class="n">s</span><span class="p">)</span>
<span class="n">_</span> <span class="o">=</span> <span class="n">StructuredMessage</span> <span class="c1"># optional, to improve readability</span>
<span class="k">def</span> <span class="nf">main</span><span class="p">():</span>
<span class="n">logging</span><span class="o">.</span><span class="n">basicConfig</span><span class="p">(</span><span class="n">level</span><span class="o">=</span><span class="n">logging</span><span class="o">.</span><span class="n">INFO</span><span class="p">,</span> <span class="nb">format</span><span class="o">=</span><span class="s1">'</span><span class="si">%(message)s</span><span class="s1">'</span><span class="p">)</span>
<span class="n">logging</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="n">_</span><span class="p">(</span><span class="s1">'message 1'</span><span class="p">,</span> <span class="n">set_value</span><span class="o">=</span><span class="p">{</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">},</span> <span class="n">snowman</span><span class="o">=</span><span class="s1">'</span><span class="se">\u2603</span><span class="s1">'</span><span class="p">))</span>
<span class="k">if</span> <span class="n">__name__</span> <span class="o">==</span> <span class="s1">'__main__'</span><span class="p">:</span>
<span class="n">main</span><span class="p">()</span>
</pre></div>
</div>
<p>When the above script is run, it prints:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">message</span> <span class="mi">1</span> <span class="o">>>></span> <span class="p">{</span><span class="s2">"snowman"</span><span class="p">:</span> <span class="s2">"</span><span class="se">\u2603</span><span class="s2">"</span><span class="p">,</span> <span class="s2">"set_value"</span><span class="p">:</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">]}</span>
</pre></div>
</div>
<p>Note that the order of items might be different according to the version of
Python used.</p>
</div>
<div class="section" id="customizing-handlers-with-dictconfig">
<span id="custom-handlers"></span><h2>Customizing handlers with <a class="reference internal" href="../library/logging.config.html#logging.config.dictConfig" title="logging.config.dictConfig"><code class="xref py py-func docutils literal"><span class="pre">dictConfig()</span></code></a><a class="headerlink" href="#customizing-handlers-with-dictconfig" title="Permalink to this headline">¶</a></h2>
<p>There are times when you want to customize logging handlers in particular ways,
and if you use <a class="reference internal" href="../library/logging.config.html#logging.config.dictConfig" title="logging.config.dictConfig"><code class="xref py py-func docutils literal"><span class="pre">dictConfig()</span></code></a> you may be able to do this without
subclassing. As an example, consider that you may want to set the ownership of a
log file. On POSIX, this is easily done using <a class="reference internal" href="../library/shutil.html#shutil.chown" title="shutil.chown"><code class="xref py py-func docutils literal"><span class="pre">shutil.chown()</span></code></a>, but the file
handlers in the stdlib don’t offer built-in support. You can customize handler
creation using a plain function such as:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">owned_file_handler</span><span class="p">(</span><span class="n">filename</span><span class="p">,</span> <span class="n">mode</span><span class="o">=</span><span class="s1">'a'</span><span class="p">,</span> <span class="n">encoding</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="n">owner</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span>
<span class="k">if</span> <span class="n">owner</span><span class="p">:</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">exists</span><span class="p">(</span><span class="n">filename</span><span class="p">):</span>
<span class="nb">open</span><span class="p">(</span><span class="n">filename</span><span class="p">,</span> <span class="s1">'a'</span><span class="p">)</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
<span class="n">shutil</span><span class="o">.</span><span class="n">chown</span><span class="p">(</span><span class="n">filename</span><span class="p">,</span> <span class="o">*</span><span class="n">owner</span><span class="p">)</span>
<span class="k">return</span> <span class="n">logging</span><span class="o">.</span><span class="n">FileHandler</span><span class="p">(</span><span class="n">filename</span><span class="p">,</span> <span class="n">mode</span><span class="p">,</span> <span class="n">encoding</span><span class="p">)</span>
</pre></div>
</div>
<p>You can then specify, in a logging configuration passed to <a class="reference internal" href="../library/logging.config.html#logging.config.dictConfig" title="logging.config.dictConfig"><code class="xref py py-func docutils literal"><span class="pre">dictConfig()</span></code></a>,
that a logging handler be created by calling this function:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">LOGGING</span> <span class="o">=</span> <span class="p">{</span>
<span class="s1">'version'</span><span class="p">:</span> <span class="mi">1</span><span class="p">,</span>
<span class="s1">'disable_existing_loggers'</span><span class="p">:</span> <span class="kc">False</span><span class="p">,</span>
<span class="s1">'formatters'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'default'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'format'</span><span class="p">:</span> <span class="s1">'</span><span class="si">%(asctime)s</span><span class="s1"> </span><span class="si">%(levelname)s</span><span class="s1"> </span><span class="si">%(name)s</span><span class="s1"> </span><span class="si">%(message)s</span><span class="s1">'</span>
<span class="p">},</span>
<span class="p">},</span>
<span class="s1">'handlers'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'file'</span><span class="p">:{</span>
<span class="c1"># The values below are popped from this dictionary and</span>
<span class="c1"># used to create the handler, set the handler's level and</span>
<span class="c1"># its formatter.</span>
<span class="s1">'()'</span><span class="p">:</span> <span class="n">owned_file_handler</span><span class="p">,</span>
<span class="s1">'level'</span><span class="p">:</span><span class="s1">'DEBUG'</span><span class="p">,</span>
<span class="s1">'formatter'</span><span class="p">:</span> <span class="s1">'default'</span><span class="p">,</span>
<span class="c1"># The values below are passed to the handler creator callable</span>
<span class="c1"># as keyword arguments.</span>
<span class="s1">'owner'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'pulse'</span><span class="p">,</span> <span class="s1">'pulse'</span><span class="p">],</span>
<span class="s1">'filename'</span><span class="p">:</span> <span class="s1">'chowntest.log'</span><span class="p">,</span>
<span class="s1">'mode'</span><span class="p">:</span> <span class="s1">'w'</span><span class="p">,</span>
<span class="s1">'encoding'</span><span class="p">:</span> <span class="s1">'utf-8'</span><span class="p">,</span>
<span class="p">},</span>
<span class="p">},</span>
<span class="s1">'root'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'handlers'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'file'</span><span class="p">],</span>
<span class="s1">'level'</span><span class="p">:</span> <span class="s1">'DEBUG'</span><span class="p">,</span>
<span class="p">},</span>
<span class="p">}</span>
</pre></div>
</div>
<p>In this example I am setting the ownership using the <code class="docutils literal"><span class="pre">pulse</span></code> user and group,
just for the purposes of illustration. Putting it together into a working
script, <code class="docutils literal"><span class="pre">chowntest.py</span></code>:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">logging</span><span class="o">,</span> <span class="nn">logging.config</span><span class="o">,</span> <span class="nn">os</span><span class="o">,</span> <span class="nn">shutil</span>
<span class="k">def</span> <span class="nf">owned_file_handler</span><span class="p">(</span><span class="n">filename</span><span class="p">,</span> <span class="n">mode</span><span class="o">=</span><span class="s1">'a'</span><span class="p">,</span> <span class="n">encoding</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="n">owner</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span>
<span class="k">if</span> <span class="n">owner</span><span class="p">:</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">exists</span><span class="p">(</span><span class="n">filename</span><span class="p">):</span>
<span class="nb">open</span><span class="p">(</span><span class="n">filename</span><span class="p">,</span> <span class="s1">'a'</span><span class="p">)</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
<span class="n">shutil</span><span class="o">.</span><span class="n">chown</span><span class="p">(</span><span class="n">filename</span><span class="p">,</span> <span class="o">*</span><span class="n">owner</span><span class="p">)</span>
<span class="k">return</span> <span class="n">logging</span><span class="o">.</span><span class="n">FileHandler</span><span class="p">(</span><span class="n">filename</span><span class="p">,</span> <span class="n">mode</span><span class="p">,</span> <span class="n">encoding</span><span class="p">)</span>
<span class="n">LOGGING</span> <span class="o">=</span> <span class="p">{</span>
<span class="s1">'version'</span><span class="p">:</span> <span class="mi">1</span><span class="p">,</span>
<span class="s1">'disable_existing_loggers'</span><span class="p">:</span> <span class="kc">False</span><span class="p">,</span>
<span class="s1">'formatters'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'default'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'format'</span><span class="p">:</span> <span class="s1">'</span><span class="si">%(asctime)s</span><span class="s1"> </span><span class="si">%(levelname)s</span><span class="s1"> </span><span class="si">%(name)s</span><span class="s1"> </span><span class="si">%(message)s</span><span class="s1">'</span>
<span class="p">},</span>
<span class="p">},</span>
<span class="s1">'handlers'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'file'</span><span class="p">:{</span>
<span class="c1"># The values below are popped from this dictionary and</span>
<span class="c1"># used to create the handler, set the handler's level and</span>
<span class="c1"># its formatter.</span>
<span class="s1">'()'</span><span class="p">:</span> <span class="n">owned_file_handler</span><span class="p">,</span>
<span class="s1">'level'</span><span class="p">:</span><span class="s1">'DEBUG'</span><span class="p">,</span>
<span class="s1">'formatter'</span><span class="p">:</span> <span class="s1">'default'</span><span class="p">,</span>
<span class="c1"># The values below are passed to the handler creator callable</span>
<span class="c1"># as keyword arguments.</span>
<span class="s1">'owner'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'pulse'</span><span class="p">,</span> <span class="s1">'pulse'</span><span class="p">],</span>
<span class="s1">'filename'</span><span class="p">:</span> <span class="s1">'chowntest.log'</span><span class="p">,</span>
<span class="s1">'mode'</span><span class="p">:</span> <span class="s1">'w'</span><span class="p">,</span>
<span class="s1">'encoding'</span><span class="p">:</span> <span class="s1">'utf-8'</span><span class="p">,</span>
<span class="p">},</span>
<span class="p">},</span>
<span class="s1">'root'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'handlers'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'file'</span><span class="p">],</span>
<span class="s1">'level'</span><span class="p">:</span> <span class="s1">'DEBUG'</span><span class="p">,</span>
<span class="p">},</span>
<span class="p">}</span>
<span class="n">logging</span><span class="o">.</span><span class="n">config</span><span class="o">.</span><span class="n">dictConfig</span><span class="p">(</span><span class="n">LOGGING</span><span class="p">)</span>
<span class="n">logger</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="s1">'mylogger'</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s1">'A debug message'</span><span class="p">)</span>
</pre></div>
</div>
<p>To run this, you will probably need to run as <code class="docutils literal"><span class="pre">root</span></code>:</p>
<div class="highlight-shell-session"><div class="highlight"><pre><span></span><span class="gp">$</span> sudo python3.3 chowntest.py
<span class="gp">$</span> cat chowntest.log
<span class="go">2013-11-05 09:34:51,128 DEBUG mylogger A debug message</span>
<span class="gp">$</span> ls -l chowntest.log
<span class="go">-rw-r--r-- 1 pulse pulse 55 2013-11-05 09:34 chowntest.log</span>
</pre></div>
</div>
<p>Note that this example uses Python 3.3 because that’s where <a class="reference internal" href="../library/shutil.html#shutil.chown" title="shutil.chown"><code class="xref py py-func docutils literal"><span class="pre">shutil.chown()</span></code></a>
makes an appearance. This approach should work with any Python version that
supports <a class="reference internal" href="../library/logging.config.html#logging.config.dictConfig" title="logging.config.dictConfig"><code class="xref py py-func docutils literal"><span class="pre">dictConfig()</span></code></a> - namely, Python 2.7, 3.2 or later. With pre-3.3
versions, you would need to implement the actual ownership change using e.g.
<a class="reference internal" href="../library/os.html#os.chown" title="os.chown"><code class="xref py py-func docutils literal"><span class="pre">os.chown()</span></code></a>.</p>
<p>In practice, the handler-creating function may be in a utility module somewhere
in your project. Instead of the line in the configuration:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="s1">'()'</span><span class="p">:</span> <span class="n">owned_file_handler</span><span class="p">,</span>
</pre></div>
</div>
<p>you could use e.g.:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="s1">'()'</span><span class="p">:</span> <span class="s1">'ext://project.util.owned_file_handler'</span><span class="p">,</span>
</pre></div>
</div>
<p>where <code class="docutils literal"><span class="pre">project.util</span></code> can be replaced with the actual name of the package
where the function resides. In the above working script, using
<code class="docutils literal"><span class="pre">'ext://__main__.owned_file_handler'</span></code> should work. Here, the actual callable
is resolved by <a class="reference internal" href="../library/logging.config.html#logging.config.dictConfig" title="logging.config.dictConfig"><code class="xref py py-func docutils literal"><span class="pre">dictConfig()</span></code></a> from the <code class="docutils literal"><span class="pre">ext://</span></code> specification.</p>
<p>This example hopefully also points the way to how you could implement other
types of file change - e.g. setting specific POSIX permission bits - in the
same way, using <a class="reference internal" href="../library/os.html#os.chmod" title="os.chmod"><code class="xref py py-func docutils literal"><span class="pre">os.chmod()</span></code></a>.</p>
<p>Of course, the approach could also be extended to types of handler other than a
<a class="reference internal" href="../library/logging.handlers.html#logging.FileHandler" title="logging.FileHandler"><code class="xref py py-class docutils literal"><span class="pre">FileHandler</span></code></a> - for example, one of the rotating file handlers,
or a different type of handler altogether.</p>
</div>
<div class="section" id="using-particular-formatting-styles-throughout-your-application">
<span id="formatting-styles"></span><h2>Using particular formatting styles throughout your application<a class="headerlink" href="#using-particular-formatting-styles-throughout-your-application" title="Permalink to this headline">¶</a></h2>
<p>In Python 3.2, the <a class="reference internal" href="../library/logging.html#logging.Formatter" title="logging.Formatter"><code class="xref py py-class docutils literal"><span class="pre">Formatter</span></code></a> gained a <code class="docutils literal"><span class="pre">style</span></code> keyword
parameter which, while defaulting to <code class="docutils literal"><span class="pre">%</span></code> for backward compatibility, allowed
the specification of <code class="docutils literal"><span class="pre">{</span></code> or <code class="docutils literal"><span class="pre">$</span></code> to support the formatting approaches
supported by <a class="reference internal" href="../library/stdtypes.html#str.format" title="str.format"><code class="xref py py-meth docutils literal"><span class="pre">str.format()</span></code></a> and <a class="reference internal" href="../library/string.html#string.Template" title="string.Template"><code class="xref py py-class docutils literal"><span class="pre">string.Template</span></code></a>. Note that this
governs the formatting of logging messages for final output to logs, and is
completely orthogonal to how an individual logging message is constructed.</p>
<p>Logging calls (<a class="reference internal" href="../library/logging.html#logging.Logger.debug" title="logging.Logger.debug"><code class="xref py py-meth docutils literal"><span class="pre">debug()</span></code></a>, <a class="reference internal" href="../library/logging.html#logging.Logger.info" title="logging.Logger.info"><code class="xref py py-meth docutils literal"><span class="pre">info()</span></code></a> etc.) only take
positional parameters for the actual logging message itself, with keyword
parameters used only for determining options for how to handle the logging call
(e.g. the <code class="docutils literal"><span class="pre">exc_info</span></code> keyword parameter to indicate that traceback information
should be logged, or the <code class="docutils literal"><span class="pre">extra</span></code> keyword parameter to indicate additional
contextual information to be added to the log). So you cannot directly make
logging calls using <a class="reference internal" href="../library/stdtypes.html#str.format" title="str.format"><code class="xref py py-meth docutils literal"><span class="pre">str.format()</span></code></a> or <a class="reference internal" href="../library/string.html#string.Template" title="string.Template"><code class="xref py py-class docutils literal"><span class="pre">string.Template</span></code></a> syntax,
because internally the logging package uses %-formatting to merge the format
string and the variable arguments. There would no changing this while preserving
backward compatibility, since all logging calls which are out there in existing
code will be using %-format strings.</p>
<p>There have been suggestions to associate format styles with specific loggers,
but that approach also runs into backward compatibility problems because any
existing code could be using a given logger name and using %-formatting.</p>
<p>For logging to work interoperably between any third-party libraries and your
code, decisions about formatting need to be made at the level of the
individual logging call. This opens up a couple of ways in which alternative
formatting styles can be accommodated.</p>
<div class="section" id="using-logrecord-factories">
<h3>Using LogRecord factories<a class="headerlink" href="#using-logrecord-factories" title="Permalink to this headline">¶</a></h3>
<p>In Python 3.2, along with the <a class="reference internal" href="../library/logging.html#logging.Formatter" title="logging.Formatter"><code class="xref py py-class docutils literal"><span class="pre">Formatter</span></code></a> changes mentioned
above, the logging package gained the ability to allow users to set their own
<a class="reference internal" href="../library/logging.html#logging.LogRecord" title="logging.LogRecord"><code class="xref py py-class docutils literal"><span class="pre">LogRecord</span></code></a> subclasses, using the <a class="reference internal" href="../library/logging.html#logging.setLogRecordFactory" title="logging.setLogRecordFactory"><code class="xref py py-func docutils literal"><span class="pre">setLogRecordFactory()</span></code></a> function.
You can use this to set your own subclass of <a class="reference internal" href="../library/logging.html#logging.LogRecord" title="logging.LogRecord"><code class="xref py py-class docutils literal"><span class="pre">LogRecord</span></code></a>, which does the
Right Thing by overriding the <a class="reference internal" href="../library/logging.html#logging.LogRecord.getMessage" title="logging.LogRecord.getMessage"><code class="xref py py-meth docutils literal"><span class="pre">getMessage()</span></code></a> method. The base
class implementation of this method is where the <code class="docutils literal"><span class="pre">msg</span> <span class="pre">%</span> <span class="pre">args</span></code> formatting
happens, and where you can substitute your alternate formatting; however, you
should be careful to support all formatting styles and allow %-formatting as
the default, to ensure interoperability with other code. Care should also be
taken to call <code class="docutils literal"><span class="pre">str(self.msg)</span></code>, just as the base implementation does.</p>
<p>Refer to the reference documentation on <a class="reference internal" href="../library/logging.html#logging.setLogRecordFactory" title="logging.setLogRecordFactory"><code class="xref py py-func docutils literal"><span class="pre">setLogRecordFactory()</span></code></a> and
<a class="reference internal" href="../library/logging.html#logging.LogRecord" title="logging.LogRecord"><code class="xref py py-class docutils literal"><span class="pre">LogRecord</span></code></a> for more information.</p>
</div>
<div class="section" id="using-custom-message-objects">
<h3>Using custom message objects<a class="headerlink" href="#using-custom-message-objects" title="Permalink to this headline">¶</a></h3>
<p>There is another, perhaps simpler way that you can use {}- and $- formatting to
construct your individual log messages. You may recall (from
<a class="reference internal" href="logging.html#arbitrary-object-messages"><span>Using arbitrary objects as messages</span></a>) that when logging you can use an arbitrary
object as a message format string, and that the logging package will call
<a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-func docutils literal"><span class="pre">str()</span></code></a> on that object to get the actual format string. Consider the
following two classes:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">BraceMessage</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">fmt</span><span class="p">,</span> <span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">fmt</span> <span class="o">=</span> <span class="n">fmt</span>
<span class="bp">self</span><span class="o">.</span><span class="n">args</span> <span class="o">=</span> <span class="n">args</span>
<span class="bp">self</span><span class="o">.</span><span class="n">kwargs</span> <span class="o">=</span> <span class="n">kwargs</span>
<span class="k">def</span> <span class="nf">__str__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">fmt</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="o">*</span><span class="bp">self</span><span class="o">.</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="bp">self</span><span class="o">.</span><span class="n">kwargs</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">DollarMessage</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">fmt</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">fmt</span> <span class="o">=</span> <span class="n">fmt</span>
<span class="bp">self</span><span class="o">.</span><span class="n">kwargs</span> <span class="o">=</span> <span class="n">kwargs</span>
<span class="k">def</span> <span class="nf">__str__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="kn">from</span> <span class="nn">string</span> <span class="k">import</span> <span class="n">Template</span>
<span class="k">return</span> <span class="n">Template</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">fmt</span><span class="p">)</span><span class="o">.</span><span class="n">substitute</span><span class="p">(</span><span class="o">**</span><span class="bp">self</span><span class="o">.</span><span class="n">kwargs</span><span class="p">)</span>
</pre></div>
</div>
<p>Either of these can be used in place of a format string, to allow {}- or
$-formatting to be used to build the actual “message” part which appears in the
formatted log output in place of “%(message)s” or “{message}” or “$message”.
If you find it a little unwieldy to use the class names whenever you want to log
something, you can make it more palatable if you use an alias such as <code class="docutils literal"><span class="pre">M</span></code> or
<code class="docutils literal"><span class="pre">_</span></code> for the message (or perhaps <code class="docutils literal"><span class="pre">__</span></code>, if you are using <code class="docutils literal"><span class="pre">_</span></code> for
localization).</p>
<p>Examples of this approach are given below. Firstly, formatting with
<a class="reference internal" href="../library/stdtypes.html#str.format" title="str.format"><code class="xref py py-meth docutils literal"><span class="pre">str.format()</span></code></a>:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">__</span> <span class="o">=</span> <span class="n">BraceMessage</span>
<span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="n">__</span><span class="p">(</span><span class="s1">'Message with </span><span class="si">{0}</span><span class="s1"> </span><span class="si">{1}</span><span class="s1">'</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="s1">'placeholders'</span><span class="p">))</span>
<span class="go">Message with 2 placeholders</span>
<span class="gp">>>> </span><span class="k">class</span> <span class="nc">Point</span><span class="p">:</span> <span class="k">pass</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="n">p</span> <span class="o">=</span> <span class="n">Point</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">p</span><span class="o">.</span><span class="n">x</span> <span class="o">=</span> <span class="mf">0.5</span>
<span class="gp">>>> </span><span class="n">p</span><span class="o">.</span><span class="n">y</span> <span class="o">=</span> <span class="mf">0.5</span>
<span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="n">__</span><span class="p">(</span><span class="s1">'Message with coordinates: (</span><span class="si">{point.x:.2f}</span><span class="s1">, </span><span class="si">{point.y:.2f}</span><span class="s1">)'</span><span class="p">,</span> <span class="n">point</span><span class="o">=</span><span class="n">p</span><span class="p">))</span>
<span class="go">Message with coordinates: (0.50, 0.50)</span>
</pre></div>
</div>
<p>Secondly, formatting with <a class="reference internal" href="../library/string.html#string.Template" title="string.Template"><code class="xref py py-class docutils literal"><span class="pre">string.Template</span></code></a>:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">__</span> <span class="o">=</span> <span class="n">DollarMessage</span>
<span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="n">__</span><span class="p">(</span><span class="s1">'Message with $num $what'</span><span class="p">,</span> <span class="n">num</span><span class="o">=</span><span class="mi">2</span><span class="p">,</span> <span class="n">what</span><span class="o">=</span><span class="s1">'placeholders'</span><span class="p">))</span>
<span class="go">Message with 2 placeholders</span>
<span class="go">>>></span>
</pre></div>
</div>
<p>One thing to note is that you pay no significant performance penalty with this
approach: the actual formatting happens not when you make the logging call, but
when (and if) the logged message is actually about to be output to a log by a
handler. So the only slightly unusual thing which might trip you up is that the
parentheses go around the format string and the arguments, not just the format
string. That’s because the __ notation is just syntax sugar for a constructor
call to one of the <code class="docutils literal"><span class="pre">XXXMessage</span></code> classes shown above.</p>
</div>
</div>
<div class="section" id="configuring-filters-with-dictconfig">
<span id="filters-dictconfig"></span><h2>Configuring filters with <a class="reference internal" href="../library/logging.config.html#logging.config.dictConfig" title="logging.config.dictConfig"><code class="xref py py-func docutils literal"><span class="pre">dictConfig()</span></code></a><a class="headerlink" href="#configuring-filters-with-dictconfig" title="Permalink to this headline">¶</a></h2>
<p>You <em>can</em> configure filters using <a class="reference internal" href="../library/logging.config.html#logging.config.dictConfig" title="logging.config.dictConfig"><code class="xref py py-func docutils literal"><span class="pre">dictConfig()</span></code></a>, though it
might not be obvious at first glance how to do it (hence this recipe). Since
<a class="reference internal" href="../library/logging.html#logging.Filter" title="logging.Filter"><code class="xref py py-class docutils literal"><span class="pre">Filter</span></code></a> is the only filter class included in the standard
library, and it is unlikely to cater to many requirements (it’s only there as a
base class), you will typically need to define your own <a class="reference internal" href="../library/logging.html#logging.Filter" title="logging.Filter"><code class="xref py py-class docutils literal"><span class="pre">Filter</span></code></a>
subclass with an overridden <a class="reference internal" href="../library/logging.html#logging.Filter.filter" title="logging.Filter.filter"><code class="xref py py-meth docutils literal"><span class="pre">filter()</span></code></a> method. To do this,
specify the <code class="docutils literal"><span class="pre">()</span></code> key in the configuration dictionary for the filter,
specifying a callable which will be used to create the filter (a class is the
most obvious, but you can provide any callable which returns a
<a class="reference internal" href="../library/logging.html#logging.Filter" title="logging.Filter"><code class="xref py py-class docutils literal"><span class="pre">Filter</span></code></a> instance). Here is a complete example:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">logging</span>
<span class="kn">import</span> <span class="nn">logging.config</span>
<span class="kn">import</span> <span class="nn">sys</span>
<span class="k">class</span> <span class="nc">MyFilter</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">Filter</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">param</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">param</span> <span class="o">=</span> <span class="n">param</span>
<span class="k">def</span> <span class="nf">filter</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">record</span><span class="p">):</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">param</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
<span class="n">allow</span> <span class="o">=</span> <span class="kc">True</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">allow</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">param</span> <span class="ow">not</span> <span class="ow">in</span> <span class="n">record</span><span class="o">.</span><span class="n">msg</span>
<span class="k">if</span> <span class="n">allow</span><span class="p">:</span>
<span class="n">record</span><span class="o">.</span><span class="n">msg</span> <span class="o">=</span> <span class="s1">'changed: '</span> <span class="o">+</span> <span class="n">record</span><span class="o">.</span><span class="n">msg</span>
<span class="k">return</span> <span class="n">allow</span>
<span class="n">LOGGING</span> <span class="o">=</span> <span class="p">{</span>
<span class="s1">'version'</span><span class="p">:</span> <span class="mi">1</span><span class="p">,</span>
<span class="s1">'filters'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'myfilter'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'()'</span><span class="p">:</span> <span class="n">MyFilter</span><span class="p">,</span>
<span class="s1">'param'</span><span class="p">:</span> <span class="s1">'noshow'</span><span class="p">,</span>
<span class="p">}</span>
<span class="p">},</span>
<span class="s1">'handlers'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'console'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'class'</span><span class="p">:</span> <span class="s1">'logging.StreamHandler'</span><span class="p">,</span>
<span class="s1">'filters'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'myfilter'</span><span class="p">]</span>
<span class="p">}</span>
<span class="p">},</span>
<span class="s1">'root'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'level'</span><span class="p">:</span> <span class="s1">'DEBUG'</span><span class="p">,</span>
<span class="s1">'handlers'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'console'</span><span class="p">]</span>
<span class="p">},</span>
<span class="p">}</span>
<span class="k">if</span> <span class="n">__name__</span> <span class="o">==</span> <span class="s1">'__main__'</span><span class="p">:</span>
<span class="n">logging</span><span class="o">.</span><span class="n">config</span><span class="o">.</span><span class="n">dictConfig</span><span class="p">(</span><span class="n">LOGGING</span><span class="p">)</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s1">'hello'</span><span class="p">)</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s1">'hello - noshow'</span><span class="p">)</span>
</pre></div>
</div>
<p>This example shows how you can pass configuration data to the callable which
constructs the instance, in the form of keyword parameters. When run, the above
script will print:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">changed</span><span class="p">:</span> <span class="n">hello</span>
</pre></div>
</div>
<p>which shows that the filter is working as configured.</p>
<p>A couple of extra points to note:</p>
<ul class="simple">
<li>If you can’t refer to the callable directly in the configuration (e.g. if it
lives in a different module, and you can’t import it directly where the
configuration dictionary is), you can use the form <code class="docutils literal"><span class="pre">ext://...</span></code> as described
in <a class="reference internal" href="../library/logging.config.html#logging-config-dict-externalobj"><span>Access to external objects</span></a>. For example, you could have used
the text <code class="docutils literal"><span class="pre">'ext://__main__.MyFilter'</span></code> instead of <code class="docutils literal"><span class="pre">MyFilter</span></code> in the above
example.</li>
<li>As well as for filters, this technique can also be used to configure custom
handlers and formatters. See <a class="reference internal" href="../library/logging.config.html#logging-config-dict-userdef"><span>User-defined objects</span></a> for more
information on how logging supports using user-defined objects in its
configuration, and see the other cookbook recipe <a class="reference internal" href="#custom-handlers"><span>Customizing handlers with dictConfig()</span></a> above.</li>
</ul>
</div>
<div class="section" id="customized-exception-formatting">
<span id="custom-format-exception"></span><h2>Customized exception formatting<a class="headerlink" href="#customized-exception-formatting" title="Permalink to this headline">¶</a></h2>
<p>There might be times when you want to do customized exception formatting - for
argument’s sake, let’s say you want exactly one line per logged event, even
when exception information is present. You can do this with a custom formatter
class, as shown in the following example:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">logging</span>
<span class="k">class</span> <span class="nc">OneLineExceptionFormatter</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">Formatter</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">formatException</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">exc_info</span><span class="p">):</span>
<span class="sd">"""</span>
<span class="sd"> Format an exception so that it prints on a single line.</span>
<span class="sd"> """</span>
<span class="n">result</span> <span class="o">=</span> <span class="nb">super</span><span class="p">(</span><span class="n">OneLineExceptionFormatter</span><span class="p">,</span> <span class="bp">self</span><span class="p">)</span><span class="o">.</span><span class="n">formatException</span><span class="p">(</span><span class="n">exc_info</span><span class="p">)</span>
<span class="k">return</span> <span class="nb">repr</span><span class="p">(</span><span class="n">result</span><span class="p">)</span> <span class="c1"># or format into one line however you want to</span>
<span class="k">def</span> <span class="nf">format</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">record</span><span class="p">):</span>
<span class="n">s</span> <span class="o">=</span> <span class="nb">super</span><span class="p">(</span><span class="n">OneLineExceptionFormatter</span><span class="p">,</span> <span class="bp">self</span><span class="p">)</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">record</span><span class="p">)</span>
<span class="k">if</span> <span class="n">record</span><span class="o">.</span><span class="n">exc_text</span><span class="p">:</span>
<span class="n">s</span> <span class="o">=</span> <span class="n">s</span><span class="o">.</span><span class="n">replace</span><span class="p">(</span><span class="s1">'</span><span class="se">\n</span><span class="s1">'</span><span class="p">,</span> <span class="s1">''</span><span class="p">)</span> <span class="o">+</span> <span class="s1">'|'</span>
<span class="k">return</span> <span class="n">s</span>
<span class="k">def</span> <span class="nf">configure_logging</span><span class="p">():</span>
<span class="n">fh</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">FileHandler</span><span class="p">(</span><span class="s1">'output.txt'</span><span class="p">,</span> <span class="s1">'w'</span><span class="p">)</span>
<span class="n">f</span> <span class="o">=</span> <span class="n">OneLineExceptionFormatter</span><span class="p">(</span><span class="s1">'</span><span class="si">%(asctime)s</span><span class="s1">|</span><span class="si">%(levelname)s</span><span class="s1">|</span><span class="si">%(message)s</span><span class="s1">|'</span><span class="p">,</span>
<span class="s1">'</span><span class="si">%d</span><span class="s1">/%m/%Y %H:%M:%S'</span><span class="p">)</span>
<span class="n">fh</span><span class="o">.</span><span class="n">setFormatter</span><span class="p">(</span><span class="n">f</span><span class="p">)</span>
<span class="n">root</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">()</span>
<span class="n">root</span><span class="o">.</span><span class="n">setLevel</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">)</span>
<span class="n">root</span><span class="o">.</span><span class="n">addHandler</span><span class="p">(</span><span class="n">fh</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">main</span><span class="p">():</span>
<span class="n">configure_logging</span><span class="p">()</span>
<span class="n">logging</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'Sample message'</span><span class="p">)</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">x</span> <span class="o">=</span> <span class="mi">1</span> <span class="o">/</span> <span class="mi">0</span>
<span class="k">except</span> <span class="ne">ZeroDivisionError</span> <span class="k">as</span> <span class="n">e</span><span class="p">:</span>
<span class="n">logging</span><span class="o">.</span><span class="n">exception</span><span class="p">(</span><span class="s1">'ZeroDivisionError: </span><span class="si">%s</span><span class="s1">'</span><span class="p">,</span> <span class="n">e</span><span class="p">)</span>
<span class="k">if</span> <span class="n">__name__</span> <span class="o">==</span> <span class="s1">'__main__'</span><span class="p">:</span>
<span class="n">main</span><span class="p">()</span>
</pre></div>
</div>
<p>When run, this produces a file with exactly two lines:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="mi">28</span><span class="o">/</span><span class="mi">01</span><span class="o">/</span><span class="mi">2015</span> <span class="mi">07</span><span class="p">:</span><span class="mi">21</span><span class="p">:</span><span class="mi">23</span><span class="o">|</span><span class="n">INFO</span><span class="o">|</span><span class="n">Sample</span> <span class="n">message</span><span class="o">|</span>
<span class="mi">28</span><span class="o">/</span><span class="mi">01</span><span class="o">/</span><span class="mi">2015</span> <span class="mi">07</span><span class="p">:</span><span class="mi">21</span><span class="p">:</span><span class="mi">23</span><span class="o">|</span><span class="n">ERROR</span><span class="o">|</span><span class="ne">ZeroDivisionError</span><span class="p">:</span> <span class="n">integer</span> <span class="n">division</span> <span class="ow">or</span> <span class="n">modulo</span> <span class="n">by</span> <span class="n">zero</span><span class="o">|</span><span class="s1">'Traceback (most recent call last):</span><span class="se">\n</span><span class="s1"> File "logtest7.py", line 30, in main</span><span class="se">\n</span><span class="s1"> x = 1 / 0</span><span class="se">\n</span><span class="s1">ZeroDivisionError: integer division or modulo by zero'</span><span class="o">|</span>
</pre></div>
</div>
<p>While the above treatment is simplistic, it points the way to how exception
information can be formatted to your liking. The <a class="reference internal" href="../library/traceback.html#module-traceback" title="traceback: Print or retrieve a stack traceback."><code class="xref py py-mod docutils literal"><span class="pre">traceback</span></code></a> module may be
helpful for more specialized needs.</p>
</div>
<div class="section" id="speaking-logging-messages">
<span id="spoken-messages"></span><h2>Speaking logging messages<a class="headerlink" href="#speaking-logging-messages" title="Permalink to this headline">¶</a></h2>
<p>There might be situations when it is desirable to have logging messages rendered
in an audible rather than a visible format. This is easy to do if you have
text-to-speech (TTS) functionality available in your system, even if it doesn’t have
a Python binding. Most TTS systems have a command line program you can run, and
this can be invoked from a handler using <a class="reference internal" href="../library/subprocess.html#module-subprocess" title="subprocess: Subprocess management."><code class="xref py py-mod docutils literal"><span class="pre">subprocess</span></code></a>. It’s assumed here
that TTS command line programs won’t expect to interact with users or take a
long time to complete, and that the frequency of logged messages will be not so
high as to swamp the user with messages, and that it’s acceptable to have the
messages spoken one at a time rather than concurrently, The example implementation
below waits for one message to be spoken before the next is processed, and this
might cause other handlers to be kept waiting. Here is a short example showing
the approach, which assumes that the <code class="docutils literal"><span class="pre">espeak</span></code> TTS package is available:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">logging</span>
<span class="kn">import</span> <span class="nn">subprocess</span>
<span class="kn">import</span> <span class="nn">sys</span>
<span class="k">class</span> <span class="nc">TTSHandler</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">Handler</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">emit</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">record</span><span class="p">):</span>
<span class="n">msg</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">record</span><span class="p">)</span>
<span class="c1"># Speak slowly in a female English voice</span>
<span class="n">cmd</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'espeak'</span><span class="p">,</span> <span class="s1">'-s150'</span><span class="p">,</span> <span class="s1">'-ven+f3'</span><span class="p">,</span> <span class="n">msg</span><span class="p">]</span>
<span class="n">p</span> <span class="o">=</span> <span class="n">subprocess</span><span class="o">.</span><span class="n">Popen</span><span class="p">(</span><span class="n">cmd</span><span class="p">,</span> <span class="n">stdout</span><span class="o">=</span><span class="n">subprocess</span><span class="o">.</span><span class="n">PIPE</span><span class="p">,</span>
<span class="n">stderr</span><span class="o">=</span><span class="n">subprocess</span><span class="o">.</span><span class="n">STDOUT</span><span class="p">)</span>
<span class="c1"># wait for the program to finish</span>
<span class="n">p</span><span class="o">.</span><span class="n">communicate</span><span class="p">()</span>
<span class="k">def</span> <span class="nf">configure_logging</span><span class="p">():</span>
<span class="n">h</span> <span class="o">=</span> <span class="n">TTSHandler</span><span class="p">()</span>
<span class="n">root</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">()</span>
<span class="n">root</span><span class="o">.</span><span class="n">addHandler</span><span class="p">(</span><span class="n">h</span><span class="p">)</span>
<span class="c1"># the default formatter just returns the message</span>
<span class="n">root</span><span class="o">.</span><span class="n">setLevel</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">main</span><span class="p">():</span>
<span class="n">logging</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'Hello'</span><span class="p">)</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s1">'Goodbye'</span><span class="p">)</span>
<span class="k">if</span> <span class="n">__name__</span> <span class="o">==</span> <span class="s1">'__main__'</span><span class="p">:</span>
<span class="n">configure_logging</span><span class="p">()</span>
<span class="n">sys</span><span class="o">.</span><span class="n">exit</span><span class="p">(</span><span class="n">main</span><span class="p">())</span>
</pre></div>
</div>
<p>When run, this script should say “Hello” and then “Goodbye” in a female voice.</p>
<p>The above approach can, of course, be adapted to other TTS systems and even
other systems altogether which can process messages via external programs run
from a command line.</p>
</div>
<div class="section" id="buffering-logging-messages-and-outputting-them-conditionally">
<span id="buffered-logging"></span><h2>Buffering logging messages and outputting them conditionally<a class="headerlink" href="#buffering-logging-messages-and-outputting-them-conditionally" title="Permalink to this headline">¶</a></h2>
<p>There might be situations where you want to log messages in a temporary area
and only output them if a certain condition occurs. For example, you may want to
start logging debug events in a function, and if the function completes without
errors, you don’t want to clutter the log with the collected debug information,
but if there is an error, you want all the debug information to be output as well
as the error.</p>
<p>Here is an example which shows how you could do this using a decorator for your
functions where you want logging to behave this way. It makes use of the
<a class="reference internal" href="../library/logging.handlers.html#logging.handlers.MemoryHandler" title="logging.handlers.MemoryHandler"><code class="xref py py-class docutils literal"><span class="pre">logging.handlers.MemoryHandler</span></code></a>, which allows buffering of logged events
until some condition occurs, at which point the buffered events are <code class="docutils literal"><span class="pre">flushed</span></code>
- passed to another handler (the <code class="docutils literal"><span class="pre">target</span></code> handler) for processing. By default,
the <code class="docutils literal"><span class="pre">MemoryHandler</span></code> flushed when its buffer gets filled up or an event whose
level is greater than or equal to a specified threshold is seen. You can use this
recipe with a more specialised subclass of <code class="docutils literal"><span class="pre">MemoryHandler</span></code> if you want custom
flushing behavior.</p>
<p>The example script has a simple function, <code class="docutils literal"><span class="pre">foo</span></code>, which just cycles through
all the logging levels, writing to <code class="docutils literal"><span class="pre">sys.stderr</span></code> to say what level it’s about
to log at, and then actually logging a message at that level. You can pass a
parameter to <code class="docutils literal"><span class="pre">foo</span></code> which, if true, will log at ERROR and CRITICAL levels -
otherwise, it only logs at DEBUG, INFO and WARNING levels.</p>
<p>The script just arranges to decorate <code class="docutils literal"><span class="pre">foo</span></code> with a decorator which will do the
conditional logging that’s required. The decorator takes a logger as a parameter
and attaches a memory handler for the duration of the call to the decorated
function. The decorator can be additionally parameterised using a target handler,
a level at which flushing should occur, and a capacity for the buffer. These
default to a <a class="reference internal" href="../library/logging.handlers.html#logging.StreamHandler" title="logging.StreamHandler"><code class="xref py py-class docutils literal"><span class="pre">StreamHandler</span></code></a> which writes to <code class="docutils literal"><span class="pre">sys.stderr</span></code>,
<code class="docutils literal"><span class="pre">logging.ERROR</span></code> and <code class="docutils literal"><span class="pre">100</span></code> respectively.</p>
<p>Here’s the script:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">logging</span>
<span class="kn">from</span> <span class="nn">logging.handlers</span> <span class="k">import</span> <span class="n">MemoryHandler</span>
<span class="kn">import</span> <span class="nn">sys</span>
<span class="n">logger</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="n">__name__</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">addHandler</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">NullHandler</span><span class="p">())</span>
<span class="k">def</span> <span class="nf">log_if_errors</span><span class="p">(</span><span class="n">logger</span><span class="p">,</span> <span class="n">target_handler</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="n">flush_level</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="n">capacity</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span>
<span class="k">if</span> <span class="n">target_handler</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
<span class="n">target_handler</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">StreamHandler</span><span class="p">()</span>
<span class="k">if</span> <span class="n">flush_level</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
<span class="n">flush_level</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">ERROR</span>
<span class="k">if</span> <span class="n">capacity</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
<span class="n">capacity</span> <span class="o">=</span> <span class="mi">100</span>
<span class="n">handler</span> <span class="o">=</span> <span class="n">MemoryHandler</span><span class="p">(</span><span class="n">capacity</span><span class="p">,</span> <span class="n">flushLevel</span><span class="o">=</span><span class="n">flush_level</span><span class="p">,</span> <span class="n">target</span><span class="o">=</span><span class="n">target_handler</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">decorator</span><span class="p">(</span><span class="n">fn</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">wrapper</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span>
<span class="n">logger</span><span class="o">.</span><span class="n">addHandler</span><span class="p">(</span><span class="n">handler</span><span class="p">)</span>
<span class="k">try</span><span class="p">:</span>
<span class="k">return</span> <span class="n">fn</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">)</span>
<span class="k">except</span> <span class="ne">Exception</span><span class="p">:</span>
<span class="n">logger</span><span class="o">.</span><span class="n">exception</span><span class="p">(</span><span class="s1">'call failed'</span><span class="p">)</span>
<span class="k">raise</span>
<span class="k">finally</span><span class="p">:</span>
<span class="nb">super</span><span class="p">(</span><span class="n">MemoryHandler</span><span class="p">,</span> <span class="n">handler</span><span class="p">)</span><span class="o">.</span><span class="n">flush</span><span class="p">()</span>
<span class="n">logger</span><span class="o">.</span><span class="n">removeHandler</span><span class="p">(</span><span class="n">handler</span><span class="p">)</span>
<span class="k">return</span> <span class="n">wrapper</span>
<span class="k">return</span> <span class="n">decorator</span>
<span class="k">def</span> <span class="nf">write_line</span><span class="p">(</span><span class="n">s</span><span class="p">):</span>
<span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="s1">'</span><span class="si">%s</span><span class="se">\n</span><span class="s1">'</span> <span class="o">%</span> <span class="n">s</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">foo</span><span class="p">(</span><span class="n">fail</span><span class="o">=</span><span class="kc">False</span><span class="p">):</span>
<span class="n">write_line</span><span class="p">(</span><span class="s1">'about to log at DEBUG ...'</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s1">'Actually logged at DEBUG'</span><span class="p">)</span>
<span class="n">write_line</span><span class="p">(</span><span class="s1">'about to log at INFO ...'</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'Actually logged at INFO'</span><span class="p">)</span>
<span class="n">write_line</span><span class="p">(</span><span class="s1">'about to log at WARNING ...'</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">warning</span><span class="p">(</span><span class="s1">'Actually logged at WARNING'</span><span class="p">)</span>
<span class="k">if</span> <span class="n">fail</span><span class="p">:</span>
<span class="n">write_line</span><span class="p">(</span><span class="s1">'about to log at ERROR ...'</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">error</span><span class="p">(</span><span class="s1">'Actually logged at ERROR'</span><span class="p">)</span>
<span class="n">write_line</span><span class="p">(</span><span class="s1">'about to log at CRITICAL ...'</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">critical</span><span class="p">(</span><span class="s1">'Actually logged at CRITICAL'</span><span class="p">)</span>
<span class="k">return</span> <span class="n">fail</span>
<span class="n">decorated_foo</span> <span class="o">=</span> <span class="n">log_if_errors</span><span class="p">(</span><span class="n">logger</span><span class="p">)(</span><span class="n">foo</span><span class="p">)</span>
<span class="k">if</span> <span class="n">__name__</span> <span class="o">==</span> <span class="s1">'__main__'</span><span class="p">:</span>
<span class="n">logger</span><span class="o">.</span><span class="n">setLevel</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">)</span>
<span class="n">write_line</span><span class="p">(</span><span class="s1">'Calling undecorated foo with False'</span><span class="p">)</span>
<span class="k">assert</span> <span class="ow">not</span> <span class="n">foo</span><span class="p">(</span><span class="kc">False</span><span class="p">)</span>
<span class="n">write_line</span><span class="p">(</span><span class="s1">'Calling undecorated foo with True'</span><span class="p">)</span>
<span class="k">assert</span> <span class="n">foo</span><span class="p">(</span><span class="kc">True</span><span class="p">)</span>
<span class="n">write_line</span><span class="p">(</span><span class="s1">'Calling decorated foo with False'</span><span class="p">)</span>
<span class="k">assert</span> <span class="ow">not</span> <span class="n">decorated_foo</span><span class="p">(</span><span class="kc">False</span><span class="p">)</span>
<span class="n">write_line</span><span class="p">(</span><span class="s1">'Calling decorated foo with True'</span><span class="p">)</span>
<span class="k">assert</span> <span class="n">decorated_foo</span><span class="p">(</span><span class="kc">True</span><span class="p">)</span>
</pre></div>
</div>
<p>When this script is run, the following output should be observed:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">Calling</span> <span class="n">undecorated</span> <span class="n">foo</span> <span class="k">with</span> <span class="kc">False</span>
<span class="n">about</span> <span class="n">to</span> <span class="n">log</span> <span class="n">at</span> <span class="n">DEBUG</span> <span class="o">...</span>
<span class="n">about</span> <span class="n">to</span> <span class="n">log</span> <span class="n">at</span> <span class="n">INFO</span> <span class="o">...</span>
<span class="n">about</span> <span class="n">to</span> <span class="n">log</span> <span class="n">at</span> <span class="n">WARNING</span> <span class="o">...</span>
<span class="n">Calling</span> <span class="n">undecorated</span> <span class="n">foo</span> <span class="k">with</span> <span class="kc">True</span>
<span class="n">about</span> <span class="n">to</span> <span class="n">log</span> <span class="n">at</span> <span class="n">DEBUG</span> <span class="o">...</span>
<span class="n">about</span> <span class="n">to</span> <span class="n">log</span> <span class="n">at</span> <span class="n">INFO</span> <span class="o">...</span>
<span class="n">about</span> <span class="n">to</span> <span class="n">log</span> <span class="n">at</span> <span class="n">WARNING</span> <span class="o">...</span>
<span class="n">about</span> <span class="n">to</span> <span class="n">log</span> <span class="n">at</span> <span class="n">ERROR</span> <span class="o">...</span>
<span class="n">about</span> <span class="n">to</span> <span class="n">log</span> <span class="n">at</span> <span class="n">CRITICAL</span> <span class="o">...</span>
<span class="n">Calling</span> <span class="n">decorated</span> <span class="n">foo</span> <span class="k">with</span> <span class="kc">False</span>
<span class="n">about</span> <span class="n">to</span> <span class="n">log</span> <span class="n">at</span> <span class="n">DEBUG</span> <span class="o">...</span>
<span class="n">about</span> <span class="n">to</span> <span class="n">log</span> <span class="n">at</span> <span class="n">INFO</span> <span class="o">...</span>
<span class="n">about</span> <span class="n">to</span> <span class="n">log</span> <span class="n">at</span> <span class="n">WARNING</span> <span class="o">...</span>
<span class="n">Calling</span> <span class="n">decorated</span> <span class="n">foo</span> <span class="k">with</span> <span class="kc">True</span>
<span class="n">about</span> <span class="n">to</span> <span class="n">log</span> <span class="n">at</span> <span class="n">DEBUG</span> <span class="o">...</span>
<span class="n">about</span> <span class="n">to</span> <span class="n">log</span> <span class="n">at</span> <span class="n">INFO</span> <span class="o">...</span>
<span class="n">about</span> <span class="n">to</span> <span class="n">log</span> <span class="n">at</span> <span class="n">WARNING</span> <span class="o">...</span>
<span class="n">about</span> <span class="n">to</span> <span class="n">log</span> <span class="n">at</span> <span class="n">ERROR</span> <span class="o">...</span>
<span class="n">Actually</span> <span class="n">logged</span> <span class="n">at</span> <span class="n">DEBUG</span>
<span class="n">Actually</span> <span class="n">logged</span> <span class="n">at</span> <span class="n">INFO</span>
<span class="n">Actually</span> <span class="n">logged</span> <span class="n">at</span> <span class="n">WARNING</span>
<span class="n">Actually</span> <span class="n">logged</span> <span class="n">at</span> <span class="n">ERROR</span>
<span class="n">about</span> <span class="n">to</span> <span class="n">log</span> <span class="n">at</span> <span class="n">CRITICAL</span> <span class="o">...</span>
<span class="n">Actually</span> <span class="n">logged</span> <span class="n">at</span> <span class="n">CRITICAL</span>
</pre></div>
</div>
<p>As you can see, actual logging output only occurs when an event is logged whose
severity is ERROR or greater, but in that case, any previous events at lower
severities are also logged.</p>
<p>You can of course use the conventional means of decoration:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="nd">@log_if_errors</span><span class="p">(</span><span class="n">logger</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">foo</span><span class="p">(</span><span class="n">fail</span><span class="o">=</span><span class="kc">False</span><span class="p">):</span>
<span class="o">...</span>
</pre></div>
</div>
</div>
<div class="section" id="formatting-times-using-utc-gmt-via-configuration">
<span id="utc-formatting"></span><h2>Formatting times using UTC (GMT) via configuration<a class="headerlink" href="#formatting-times-using-utc-gmt-via-configuration" title="Permalink to this headline">¶</a></h2>
<p>Sometimes you want to format times using UTC, which can be done using a class
such as <cite>UTCFormatter</cite>, shown below:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">logging</span>
<span class="kn">import</span> <span class="nn">time</span>
<span class="k">class</span> <span class="nc">UTCFormatter</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">Formatter</span><span class="p">):</span>
<span class="n">converter</span> <span class="o">=</span> <span class="n">time</span><span class="o">.</span><span class="n">gmtime</span>
</pre></div>
</div>
<p>and you can then use the <code class="docutils literal"><span class="pre">UTCFormatter</span></code> in your code instead of
<a class="reference internal" href="../library/logging.html#logging.Formatter" title="logging.Formatter"><code class="xref py py-class docutils literal"><span class="pre">Formatter</span></code></a>. If you want to do that via configuration, you can
use the <a class="reference internal" href="../library/logging.config.html#logging.config.dictConfig" title="logging.config.dictConfig"><code class="xref py py-func docutils literal"><span class="pre">dictConfig()</span></code></a> API with an approach illustrated by
the following complete example:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">logging</span>
<span class="kn">import</span> <span class="nn">logging.config</span>
<span class="kn">import</span> <span class="nn">time</span>
<span class="k">class</span> <span class="nc">UTCFormatter</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">Formatter</span><span class="p">):</span>
<span class="n">converter</span> <span class="o">=</span> <span class="n">time</span><span class="o">.</span><span class="n">gmtime</span>
<span class="n">LOGGING</span> <span class="o">=</span> <span class="p">{</span>
<span class="s1">'version'</span><span class="p">:</span> <span class="mi">1</span><span class="p">,</span>
<span class="s1">'disable_existing_loggers'</span><span class="p">:</span> <span class="kc">False</span><span class="p">,</span>
<span class="s1">'formatters'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'utc'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'()'</span><span class="p">:</span> <span class="n">UTCFormatter</span><span class="p">,</span>
<span class="s1">'format'</span><span class="p">:</span> <span class="s1">'</span><span class="si">%(asctime)s</span><span class="s1"> </span><span class="si">%(message)s</span><span class="s1">'</span><span class="p">,</span>
<span class="p">},</span>
<span class="s1">'local'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'format'</span><span class="p">:</span> <span class="s1">'</span><span class="si">%(asctime)s</span><span class="s1"> </span><span class="si">%(message)s</span><span class="s1">'</span><span class="p">,</span>
<span class="p">}</span>
<span class="p">},</span>
<span class="s1">'handlers'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'console1'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'class'</span><span class="p">:</span> <span class="s1">'logging.StreamHandler'</span><span class="p">,</span>
<span class="s1">'formatter'</span><span class="p">:</span> <span class="s1">'utc'</span><span class="p">,</span>
<span class="p">},</span>
<span class="s1">'console2'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'class'</span><span class="p">:</span> <span class="s1">'logging.StreamHandler'</span><span class="p">,</span>
<span class="s1">'formatter'</span><span class="p">:</span> <span class="s1">'local'</span><span class="p">,</span>
<span class="p">},</span>
<span class="p">},</span>
<span class="s1">'root'</span><span class="p">:</span> <span class="p">{</span>
<span class="s1">'handlers'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'console1'</span><span class="p">,</span> <span class="s1">'console2'</span><span class="p">],</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="k">if</span> <span class="n">__name__</span> <span class="o">==</span> <span class="s1">'__main__'</span><span class="p">:</span>
<span class="n">logging</span><span class="o">.</span><span class="n">config</span><span class="o">.</span><span class="n">dictConfig</span><span class="p">(</span><span class="n">LOGGING</span><span class="p">)</span>
<span class="n">logging</span><span class="o">.</span><span class="n">warning</span><span class="p">(</span><span class="s1">'The local time is </span><span class="si">%s</span><span class="s1">'</span><span class="p">,</span> <span class="n">time</span><span class="o">.</span><span class="n">asctime</span><span class="p">())</span>
</pre></div>
</div>
<p>When this script is run, it should print something like:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="mi">2015</span><span class="o">-</span><span class="mi">10</span><span class="o">-</span><span class="mi">17</span> <span class="mi">12</span><span class="p">:</span><span class="mi">53</span><span class="p">:</span><span class="mi">29</span><span class="p">,</span><span class="mi">501</span> <span class="n">The</span> <span class="n">local</span> <span class="n">time</span> <span class="ow">is</span> <span class="n">Sat</span> <span class="n">Oct</span> <span class="mi">17</span> <span class="mi">13</span><span class="p">:</span><span class="mi">53</span><span class="p">:</span><span class="mi">29</span> <span class="mi">2015</span>
<span class="mi">2015</span><span class="o">-</span><span class="mi">10</span><span class="o">-</span><span class="mi">17</span> <span class="mi">13</span><span class="p">:</span><span class="mi">53</span><span class="p">:</span><span class="mi">29</span><span class="p">,</span><span class="mi">501</span> <span class="n">The</span> <span class="n">local</span> <span class="n">time</span> <span class="ow">is</span> <span class="n">Sat</span> <span class="n">Oct</span> <span class="mi">17</span> <span class="mi">13</span><span class="p">:</span><span class="mi">53</span><span class="p">:</span><span class="mi">29</span> <span class="mi">2015</span>
</pre></div>
</div>
<p>showing how the time is formatted both as local time and UTC, one for each
handler.</p>
</div>
<div class="section" id="using-a-context-manager-for-selective-logging">
<span id="context-manager"></span><h2>Using a context manager for selective logging<a class="headerlink" href="#using-a-context-manager-for-selective-logging" title="Permalink to this headline">¶</a></h2>
<p>There are times when it would be useful to temporarily change the logging
configuration and revert it back after doing something. For this, a context
manager is the most obvious way of saving and restoring the logging context.
Here is a simple example of such a context manager, which allows you to
optionally change the logging level and add a logging handler purely in the
scope of the context manager:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">logging</span>
<span class="kn">import</span> <span class="nn">sys</span>
<span class="k">class</span> <span class="nc">LoggingContext</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">logger</span><span class="p">,</span> <span class="n">level</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="n">handler</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="n">close</span><span class="o">=</span><span class="kc">True</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">logger</span> <span class="o">=</span> <span class="n">logger</span>
<span class="bp">self</span><span class="o">.</span><span class="n">level</span> <span class="o">=</span> <span class="n">level</span>
<span class="bp">self</span><span class="o">.</span><span class="n">handler</span> <span class="o">=</span> <span class="n">handler</span>
<span class="bp">self</span><span class="o">.</span><span class="n">close</span> <span class="o">=</span> <span class="n">close</span>
<span class="k">def</span> <span class="nf">__enter__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">level</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">old_level</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">logger</span><span class="o">.</span><span class="n">level</span>
<span class="bp">self</span><span class="o">.</span><span class="n">logger</span><span class="o">.</span><span class="n">setLevel</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">level</span><span class="p">)</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">handler</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">logger</span><span class="o">.</span><span class="n">addHandler</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">handler</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">__exit__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">et</span><span class="p">,</span> <span class="n">ev</span><span class="p">,</span> <span class="n">tb</span><span class="p">):</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">level</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">logger</span><span class="o">.</span><span class="n">setLevel</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">old_level</span><span class="p">)</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">handler</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">logger</span><span class="o">.</span><span class="n">removeHandler</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">handler</span><span class="p">)</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">handler</span> <span class="ow">and</span> <span class="bp">self</span><span class="o">.</span><span class="n">close</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">handler</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
<span class="c1"># implicit return of None => don't swallow exceptions</span>
</pre></div>
</div>
<p>If you specify a level value, the logger’s level is set to that value in the
scope of the with block covered by the context manager. If you specify a
handler, it is added to the logger on entry to the block and removed on exit
from the block. You can also ask the manager to close the handler for you on
block exit - you could do this if you don’t need the handler any more.</p>
<p>To illustrate how it works, we can add the following block of code to the
above:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="n">__name__</span> <span class="o">==</span> <span class="s1">'__main__'</span><span class="p">:</span>
<span class="n">logger</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="s1">'foo'</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">addHandler</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">StreamHandler</span><span class="p">())</span>
<span class="n">logger</span><span class="o">.</span><span class="n">setLevel</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">INFO</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'1. This should appear just once on stderr.'</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s1">'2. This should not appear.'</span><span class="p">)</span>
<span class="k">with</span> <span class="n">LoggingContext</span><span class="p">(</span><span class="n">logger</span><span class="p">,</span> <span class="n">level</span><span class="o">=</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">):</span>
<span class="n">logger</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s1">'3. This should appear once on stderr.'</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s1">'4. This should not appear.'</span><span class="p">)</span>
<span class="n">h</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">StreamHandler</span><span class="p">(</span><span class="n">sys</span><span class="o">.</span><span class="n">stdout</span><span class="p">)</span>
<span class="k">with</span> <span class="n">LoggingContext</span><span class="p">(</span><span class="n">logger</span><span class="p">,</span> <span class="n">level</span><span class="o">=</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">,</span> <span class="n">handler</span><span class="o">=</span><span class="n">h</span><span class="p">,</span> <span class="n">close</span><span class="o">=</span><span class="kc">True</span><span class="p">):</span>
<span class="n">logger</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s1">'5. This should appear twice - once on stderr and once on stdout.'</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'6. This should appear just once on stderr.'</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s1">'7. This should not appear.'</span><span class="p">)</span>
</pre></div>
</div>
<p>We initially set the logger’s level to <code class="docutils literal"><span class="pre">INFO</span></code>, so message #1 appears and
message #2 doesn’t. We then change the level to <code class="docutils literal"><span class="pre">DEBUG</span></code> temporarily in the
following <code class="docutils literal"><span class="pre">with</span></code> block, and so message #3 appears. After the block exits, the
logger’s level is restored to <code class="docutils literal"><span class="pre">INFO</span></code> and so message #4 doesn’t appear. In the
next <code class="docutils literal"><span class="pre">with</span></code> block, we set the level to <code class="docutils literal"><span class="pre">DEBUG</span></code> again but also add a handler
writing to <code class="docutils literal"><span class="pre">sys.stdout</span></code>. Thus, message #5 appears twice on the console (once
via <code class="docutils literal"><span class="pre">stderr</span></code> and once via <code class="docutils literal"><span class="pre">stdout</span></code>). After the <code class="docutils literal"><span class="pre">with</span></code> statement’s
completion, the status is as it was before so message #6 appears (like message
#1) whereas message #7 doesn’t (just like message #2).</p>
<p>If we run the resulting script, the result is as follows:</p>
<div class="highlight-shell-session"><div class="highlight"><pre><span></span><span class="gp">$</span> python logctx.py
<span class="go">1. This should appear just once on stderr.</span>
<span class="go">3. This should appear once on stderr.</span>
<span class="go">5. This should appear twice - once on stderr and once on stdout.</span>
<span class="go">5. This should appear twice - once on stderr and once on stdout.</span>
<span class="go">6. This should appear just once on stderr.</span>
</pre></div>
</div>
<p>If we run it again, but pipe <code class="docutils literal"><span class="pre">stderr</span></code> to <code class="docutils literal"><span class="pre">/dev/null</span></code>, we see the following,
which is the only message written to <code class="docutils literal"><span class="pre">stdout</span></code>:</p>
<div class="highlight-shell-session"><div class="highlight"><pre><span></span><span class="gp">$</span> python logctx.py 2>/dev/null
<span class="go">5. This should appear twice - once on stderr and once on stdout.</span>
</pre></div>
</div>
<p>Once again, but piping <code class="docutils literal"><span class="pre">stdout</span></code> to <code class="docutils literal"><span class="pre">/dev/null</span></code>, we get:</p>
<div class="highlight-shell-session"><div class="highlight"><pre><span></span><span class="gp">$</span> python logctx.py >/dev/null
<span class="go">1. This should appear just once on stderr.</span>
<span class="go">3. This should appear once on stderr.</span>
<span class="go">5. This should appear twice - once on stderr and once on stdout.</span>
<span class="go">6. This should appear just once on stderr.</span>
</pre></div>
</div>
<p>In this case, the message #5 printed to <code class="docutils literal"><span class="pre">stdout</span></code> doesn’t appear, as expected.</p>
<p>Of course, the approach described here can be generalised, for example to attach
logging filters temporarily. Note that the above code works in Python 2 as well
as Python 3.</p>
</div>
</div>
</div>
</div>
</div>
<div class="sphinxsidebar" role="navigation" aria-label="main navigation">
<div class="sphinxsidebarwrapper">
<h3><a href="../contents.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">Logging Cookbook</a><ul>
<li><a class="reference internal" href="#using-logging-in-multiple-modules">Using logging in multiple modules</a></li>
<li><a class="reference internal" href="#logging-from-multiple-threads">Logging from multiple threads</a></li>
<li><a class="reference internal" href="#multiple-handlers-and-formatters">Multiple handlers and formatters</a></li>
<li><a class="reference internal" href="#logging-to-multiple-destinations">Logging to multiple destinations</a></li>
<li><a class="reference internal" href="#configuration-server-example">Configuration server example</a></li>
<li><a class="reference internal" href="#dealing-with-handlers-that-block">Dealing with handlers that block</a></li>
<li><a class="reference internal" href="#sending-and-receiving-logging-events-across-a-network">Sending and receiving logging events across a network</a></li>
<li><a class="reference internal" href="#adding-contextual-information-to-your-logging-output">Adding contextual information to your logging output</a><ul>
<li><a class="reference internal" href="#using-loggeradapters-to-impart-contextual-information">Using LoggerAdapters to impart contextual information</a><ul>
<li><a class="reference internal" href="#using-objects-other-than-dicts-to-pass-contextual-information">Using objects other than dicts to pass contextual information</a></li>
</ul>
</li>
<li><a class="reference internal" href="#using-filters-to-impart-contextual-information">Using Filters to impart contextual information</a></li>
</ul>
</li>
<li><a class="reference internal" href="#logging-to-a-single-file-from-multiple-processes">Logging to a single file from multiple processes</a></li>
<li><a class="reference internal" href="#using-file-rotation">Using file rotation</a></li>
<li><a class="reference internal" href="#use-of-alternative-formatting-styles">Use of alternative formatting styles</a></li>
<li><a class="reference internal" href="#customizing-logrecord">Customizing <code class="docutils literal"><span class="pre">LogRecord</span></code></a></li>
<li><a class="reference internal" href="#subclassing-queuehandler-a-zeromq-example">Subclassing QueueHandler - a ZeroMQ example</a></li>
<li><a class="reference internal" href="#subclassing-queuelistener-a-zeromq-example">Subclassing QueueListener - a ZeroMQ example</a></li>
<li><a class="reference internal" href="#an-example-dictionary-based-configuration">An example dictionary-based configuration</a></li>
<li><a class="reference internal" href="#using-a-rotator-and-namer-to-customize-log-rotation-processing">Using a rotator and namer to customize log rotation processing</a></li>
<li><a class="reference internal" href="#a-more-elaborate-multiprocessing-example">A more elaborate multiprocessing example</a></li>
<li><a class="reference internal" href="#inserting-a-bom-into-messages-sent-to-a-sysloghandler">Inserting a BOM into messages sent to a SysLogHandler</a></li>
<li><a class="reference internal" href="#implementing-structured-logging">Implementing structured logging</a></li>
<li><a class="reference internal" href="#customizing-handlers-with-dictconfig">Customizing handlers with <code class="docutils literal"><span class="pre">dictConfig()</span></code></a></li>
<li><a class="reference internal" href="#using-particular-formatting-styles-throughout-your-application">Using particular formatting styles throughout your application</a><ul>
<li><a class="reference internal" href="#using-logrecord-factories">Using LogRecord factories</a></li>
<li><a class="reference internal" href="#using-custom-message-objects">Using custom message objects</a></li>
</ul>
</li>
<li><a class="reference internal" href="#configuring-filters-with-dictconfig">Configuring filters with <code class="docutils literal"><span class="pre">dictConfig()</span></code></a></li>
<li><a class="reference internal" href="#customized-exception-formatting">Customized exception formatting</a></li>
<li><a class="reference internal" href="#speaking-logging-messages">Speaking logging messages</a></li>
<li><a class="reference internal" href="#buffering-logging-messages-and-outputting-them-conditionally">Buffering logging messages and outputting them conditionally</a></li>
<li><a class="reference internal" href="#formatting-times-using-utc-gmt-via-configuration">Formatting times using UTC (GMT) via configuration</a></li>
<li><a class="reference internal" href="#using-a-context-manager-for-selective-logging">Using a context manager for selective logging</a></li>
</ul>
</li>
</ul>
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