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<div class="section" id="module-contextlib">
<span id="contextlib-utilities-for-with-statement-contexts"></span><h1><code class="xref py py-mod docutils literal notranslate"><span class="pre">contextlib</span></code> — Utilities for <code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code>-statement contexts<a class="headerlink" href="#module-contextlib" title="Permalink to this headline">¶</a></h1>
<p><strong>Source code:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/3.7/Lib/contextlib.py">Lib/contextlib.py</a></p>
<hr class="docutils" />
<p>This module provides utilities for common tasks involving the <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a>
statement. For more information see also <a class="reference internal" href="stdtypes.html#typecontextmanager"><span class="std std-ref">Context Manager Types</span></a> and
<a class="reference internal" href="../reference/datamodel.html#context-managers"><span class="std std-ref">With Statement Context Managers</span></a>.</p>
<div class="section" id="utilities">
<h2>Utilities<a class="headerlink" href="#utilities" title="Permalink to this headline">¶</a></h2>
<p>Functions and classes provided:</p>
<dl class="class">
<dt id="contextlib.AbstractContextManager">
<em class="property">class </em><code class="sig-prename descclassname">contextlib.</code><code class="sig-name descname">AbstractContextManager</code><a class="headerlink" href="#contextlib.AbstractContextManager" title="Permalink to this definition">¶</a></dt>
<dd><p>An <a class="reference internal" href="../glossary.html#term-abstract-base-class"><span class="xref std std-term">abstract base class</span></a> for classes that implement
<a class="reference internal" href="../reference/datamodel.html#object.__enter__" title="object.__enter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">object.__enter__()</span></code></a> and <a class="reference internal" href="../reference/datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">object.__exit__()</span></code></a>. A default
implementation for <a class="reference internal" href="../reference/datamodel.html#object.__enter__" title="object.__enter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">object.__enter__()</span></code></a> is provided which returns
<code class="docutils literal notranslate"><span class="pre">self</span></code> while <a class="reference internal" href="../reference/datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">object.__exit__()</span></code></a> is an abstract method which by default
returns <code class="docutils literal notranslate"><span class="pre">None</span></code>. See also the definition of <a class="reference internal" href="stdtypes.html#typecontextmanager"><span class="std std-ref">Context Manager Types</span></a>.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.6.</span></p>
</div>
</dd></dl>
<dl class="class">
<dt id="contextlib.AbstractAsyncContextManager">
<em class="property">class </em><code class="sig-prename descclassname">contextlib.</code><code class="sig-name descname">AbstractAsyncContextManager</code><a class="headerlink" href="#contextlib.AbstractAsyncContextManager" title="Permalink to this definition">¶</a></dt>
<dd><p>An <a class="reference internal" href="../glossary.html#term-abstract-base-class"><span class="xref std std-term">abstract base class</span></a> for classes that implement
<a class="reference internal" href="../reference/datamodel.html#object.__aenter__" title="object.__aenter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">object.__aenter__()</span></code></a> and <a class="reference internal" href="../reference/datamodel.html#object.__aexit__" title="object.__aexit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">object.__aexit__()</span></code></a>. A default
implementation for <a class="reference internal" href="../reference/datamodel.html#object.__aenter__" title="object.__aenter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">object.__aenter__()</span></code></a> is provided which returns
<code class="docutils literal notranslate"><span class="pre">self</span></code> while <a class="reference internal" href="../reference/datamodel.html#object.__aexit__" title="object.__aexit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">object.__aexit__()</span></code></a> is an abstract method which by default
returns <code class="docutils literal notranslate"><span class="pre">None</span></code>. See also the definition of
<a class="reference internal" href="../reference/datamodel.html#async-context-managers"><span class="std std-ref">Asynchronous Context Managers</span></a>.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.7.</span></p>
</div>
</dd></dl>
<dl class="function">
<dt id="contextlib.contextmanager">
<code class="sig-prename descclassname">@</code><code class="sig-prename descclassname">contextlib.</code><code class="sig-name descname">contextmanager</code><a class="headerlink" href="#contextlib.contextmanager" title="Permalink to this definition">¶</a></dt>
<dd><p>This function is a <a class="reference internal" href="../glossary.html#term-decorator"><span class="xref std std-term">decorator</span></a> that can be used to define a factory
function for <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement context managers, without needing to
create a class or separate <a class="reference internal" href="../reference/datamodel.html#object.__enter__" title="object.__enter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__enter__()</span></code></a> and <a class="reference internal" href="../reference/datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__exit__()</span></code></a> methods.</p>
<p>While many objects natively support use in with statements, sometimes a
resource needs to be managed that isn’t a context manager in its own right,
and doesn’t implement a <code class="docutils literal notranslate"><span class="pre">close()</span></code> method for use with <code class="docutils literal notranslate"><span class="pre">contextlib.closing</span></code></p>
<p>An abstract example would be the following to ensure correct resource
management:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">contextmanager</span>
<span class="nd">@contextmanager</span>
<span class="k">def</span> <span class="nf">managed_resource</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">kwds</span><span class="p">):</span>
<span class="c1"># Code to acquire resource, e.g.:</span>
<span class="n">resource</span> <span class="o">=</span> <span class="n">acquire_resource</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">kwds</span><span class="p">)</span>
<span class="k">try</span><span class="p">:</span>
<span class="k">yield</span> <span class="n">resource</span>
<span class="k">finally</span><span class="p">:</span>
<span class="c1"># Code to release resource, e.g.:</span>
<span class="n">release_resource</span><span class="p">(</span><span class="n">resource</span><span class="p">)</span>
<span class="o">>>></span> <span class="k">with</span> <span class="n">managed_resource</span><span class="p">(</span><span class="n">timeout</span><span class="o">=</span><span class="mi">3600</span><span class="p">)</span> <span class="k">as</span> <span class="n">resource</span><span class="p">:</span>
<span class="o">...</span> <span class="c1"># Resource is released at the end of this block,</span>
<span class="o">...</span> <span class="c1"># even if code in the block raises an exception</span>
</pre></div>
</div>
<p>The function being decorated must return a <a class="reference internal" href="../glossary.html#term-generator"><span class="xref std std-term">generator</span></a>-iterator when
called. This iterator must yield exactly one value, which will be bound to
the targets in the <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement’s <code class="xref std std-keyword docutils literal notranslate"><span class="pre">as</span></code> clause, if any.</p>
<p>At the point where the generator yields, the block nested in the <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a>
statement is executed. The generator is then resumed after the block is exited.
If an unhandled exception occurs in the block, it is reraised inside the
generator at the point where the yield occurred. Thus, you can use a
<a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a>…<a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a>…<a class="reference internal" href="../reference/compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> statement to trap
the error (if any), or ensure that some cleanup takes place. If an exception is
trapped merely in order to log it or to perform some action (rather than to
suppress it entirely), the generator must reraise that exception. Otherwise the
generator context manager will indicate to the <code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code> statement that
the exception has been handled, and execution will resume with the statement
immediately following the <code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code> statement.</p>
<p><a class="reference internal" href="#contextlib.contextmanager" title="contextlib.contextmanager"><code class="xref py py-func docutils literal notranslate"><span class="pre">contextmanager()</span></code></a> uses <a class="reference internal" href="#contextlib.ContextDecorator" title="contextlib.ContextDecorator"><code class="xref py py-class docutils literal notranslate"><span class="pre">ContextDecorator</span></code></a> so the context managers
it creates can be used as decorators as well as in <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statements.
When used as a decorator, a new generator instance is implicitly created on
each function call (this allows the otherwise “one-shot” context managers
created by <a class="reference internal" href="#contextlib.contextmanager" title="contextlib.contextmanager"><code class="xref py py-func docutils literal notranslate"><span class="pre">contextmanager()</span></code></a> to meet the requirement that context
managers support multiple invocations in order to be used as decorators).</p>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.2: </span>Use of <a class="reference internal" href="#contextlib.ContextDecorator" title="contextlib.ContextDecorator"><code class="xref py py-class docutils literal notranslate"><span class="pre">ContextDecorator</span></code></a>.</p>
</div>
</dd></dl>
<dl class="function">
<dt id="contextlib.asynccontextmanager">
<code class="sig-prename descclassname">@</code><code class="sig-prename descclassname">contextlib.</code><code class="sig-name descname">asynccontextmanager</code><a class="headerlink" href="#contextlib.asynccontextmanager" title="Permalink to this definition">¶</a></dt>
<dd><p>Similar to <a class="reference internal" href="#contextlib.contextmanager" title="contextlib.contextmanager"><code class="xref py py-func docutils literal notranslate"><span class="pre">contextmanager()</span></code></a>, but creates an
<a class="reference internal" href="../reference/datamodel.html#async-context-managers"><span class="std std-ref">asynchronous context manager</span></a>.</p>
<p>This function is a <a class="reference internal" href="../glossary.html#term-decorator"><span class="xref std std-term">decorator</span></a> that can be used to define a factory
function for <a class="reference internal" href="../reference/compound_stmts.html#async-with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">async</span> <span class="pre">with</span></code></a> statement asynchronous context managers,
without needing to create a class or separate <a class="reference internal" href="../reference/datamodel.html#object.__aenter__" title="object.__aenter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__aenter__()</span></code></a> and
<a class="reference internal" href="../reference/datamodel.html#object.__aexit__" title="object.__aexit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__aexit__()</span></code></a> methods. It must be applied to an <a class="reference internal" href="../glossary.html#term-asynchronous-generator"><span class="xref std std-term">asynchronous
generator</span></a> function.</p>
<p>A simple example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">asynccontextmanager</span>
<span class="nd">@asynccontextmanager</span>
<span class="k">async</span> <span class="k">def</span> <span class="nf">get_connection</span><span class="p">():</span>
<span class="n">conn</span> <span class="o">=</span> <span class="k">await</span> <span class="n">acquire_db_connection</span><span class="p">()</span>
<span class="k">try</span><span class="p">:</span>
<span class="k">yield</span> <span class="n">conn</span>
<span class="k">finally</span><span class="p">:</span>
<span class="k">await</span> <span class="n">release_db_connection</span><span class="p">(</span><span class="n">conn</span><span class="p">)</span>
<span class="k">async</span> <span class="k">def</span> <span class="nf">get_all_users</span><span class="p">():</span>
<span class="k">async</span> <span class="k">with</span> <span class="n">get_connection</span><span class="p">()</span> <span class="k">as</span> <span class="n">conn</span><span class="p">:</span>
<span class="k">return</span> <span class="n">conn</span><span class="o">.</span><span class="n">query</span><span class="p">(</span><span class="s1">'SELECT ...'</span><span class="p">)</span>
</pre></div>
</div>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.7.</span></p>
</div>
</dd></dl>
<dl class="function">
<dt id="contextlib.closing">
<code class="sig-prename descclassname">contextlib.</code><code class="sig-name descname">closing</code><span class="sig-paren">(</span><em class="sig-param">thing</em><span class="sig-paren">)</span><a class="headerlink" href="#contextlib.closing" title="Permalink to this definition">¶</a></dt>
<dd><p>Return a context manager that closes <em>thing</em> upon completion of the block. This
is basically equivalent to:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">contextmanager</span>
<span class="nd">@contextmanager</span>
<span class="k">def</span> <span class="nf">closing</span><span class="p">(</span><span class="n">thing</span><span class="p">):</span>
<span class="k">try</span><span class="p">:</span>
<span class="k">yield</span> <span class="n">thing</span>
<span class="k">finally</span><span class="p">:</span>
<span class="n">thing</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
</pre></div>
</div>
<p>And lets you write code like this:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">closing</span>
<span class="kn">from</span> <span class="nn">urllib.request</span> <span class="kn">import</span> <span class="n">urlopen</span>
<span class="k">with</span> <span class="n">closing</span><span class="p">(</span><span class="n">urlopen</span><span class="p">(</span><span class="s1">'http://www.python.org'</span><span class="p">))</span> <span class="k">as</span> <span class="n">page</span><span class="p">:</span>
<span class="k">for</span> <span class="n">line</span> <span class="ow">in</span> <span class="n">page</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="n">line</span><span class="p">)</span>
</pre></div>
</div>
<p>without needing to explicitly close <code class="docutils literal notranslate"><span class="pre">page</span></code>. Even if an error occurs,
<code class="docutils literal notranslate"><span class="pre">page.close()</span></code> will be called when the <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> block is exited.</p>
</dd></dl>
<span class="target" id="simplifying-support-for-single-optional-context-managers"></span><dl class="function">
<dt id="contextlib.nullcontext">
<code class="sig-prename descclassname">contextlib.</code><code class="sig-name descname">nullcontext</code><span class="sig-paren">(</span><em class="sig-param">enter_result=None</em><span class="sig-paren">)</span><a class="headerlink" href="#contextlib.nullcontext" title="Permalink to this definition">¶</a></dt>
<dd><p>Return a context manager that returns <em>enter_result</em> from <code class="docutils literal notranslate"><span class="pre">__enter__</span></code>, but
otherwise does nothing. It is intended to be used as a stand-in for an
optional context manager, for example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">myfunction</span><span class="p">(</span><span class="n">arg</span><span class="p">,</span> <span class="n">ignore_exceptions</span><span class="o">=</span><span class="kc">False</span><span class="p">):</span>
<span class="k">if</span> <span class="n">ignore_exceptions</span><span class="p">:</span>
<span class="c1"># Use suppress to ignore all exceptions.</span>
<span class="n">cm</span> <span class="o">=</span> <span class="n">contextlib</span><span class="o">.</span><span class="n">suppress</span><span class="p">(</span><span class="ne">Exception</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
<span class="c1"># Do not ignore any exceptions, cm has no effect.</span>
<span class="n">cm</span> <span class="o">=</span> <span class="n">contextlib</span><span class="o">.</span><span class="n">nullcontext</span><span class="p">()</span>
<span class="k">with</span> <span class="n">cm</span><span class="p">:</span>
<span class="c1"># Do something</span>
</pre></div>
</div>
<p>An example using <em>enter_result</em>:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">process_file</span><span class="p">(</span><span class="n">file_or_path</span><span class="p">):</span>
<span class="k">if</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">file_or_path</span><span class="p">,</span> <span class="nb">str</span><span class="p">):</span>
<span class="c1"># If string, open file</span>
<span class="n">cm</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="n">file_or_path</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
<span class="c1"># Caller is responsible for closing file</span>
<span class="n">cm</span> <span class="o">=</span> <span class="n">nullcontext</span><span class="p">(</span><span class="n">file_or_path</span><span class="p">)</span>
<span class="k">with</span> <span class="n">cm</span> <span class="k">as</span> <span class="n">file</span><span class="p">:</span>
<span class="c1"># Perform processing on the file</span>
</pre></div>
</div>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.7.</span></p>
</div>
</dd></dl>
<dl class="function">
<dt id="contextlib.suppress">
<code class="sig-prename descclassname">contextlib.</code><code class="sig-name descname">suppress</code><span class="sig-paren">(</span><em class="sig-param">*exceptions</em><span class="sig-paren">)</span><a class="headerlink" href="#contextlib.suppress" title="Permalink to this definition">¶</a></dt>
<dd><p>Return a context manager that suppresses any of the specified exceptions
if they occur in the body of a with statement and then resumes execution
with the first statement following the end of the with statement.</p>
<p>As with any other mechanism that completely suppresses exceptions, this
context manager should be used only to cover very specific errors where
silently continuing with program execution is known to be the right
thing to do.</p>
<p>For example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">suppress</span>
<span class="k">with</span> <span class="n">suppress</span><span class="p">(</span><span class="ne">FileNotFoundError</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="s1">'somefile.tmp'</span><span class="p">)</span>
<span class="k">with</span> <span class="n">suppress</span><span class="p">(</span><span class="ne">FileNotFoundError</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="s1">'someotherfile.tmp'</span><span class="p">)</span>
</pre></div>
</div>
<p>This code is equivalent to:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">try</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="s1">'somefile.tmp'</span><span class="p">)</span>
<span class="k">except</span> <span class="ne">FileNotFoundError</span><span class="p">:</span>
<span class="k">pass</span>
<span class="k">try</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="s1">'someotherfile.tmp'</span><span class="p">)</span>
<span class="k">except</span> <span class="ne">FileNotFoundError</span><span class="p">:</span>
<span class="k">pass</span>
</pre></div>
</div>
<p>This context manager is <a class="reference internal" href="#reentrant-cms"><span class="std std-ref">reentrant</span></a>.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.4.</span></p>
</div>
</dd></dl>
<dl class="function">
<dt id="contextlib.redirect_stdout">
<code class="sig-prename descclassname">contextlib.</code><code class="sig-name descname">redirect_stdout</code><span class="sig-paren">(</span><em class="sig-param">new_target</em><span class="sig-paren">)</span><a class="headerlink" href="#contextlib.redirect_stdout" title="Permalink to this definition">¶</a></dt>
<dd><p>Context manager for temporarily redirecting <a class="reference internal" href="sys.html#sys.stdout" title="sys.stdout"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.stdout</span></code></a> to
another file or file-like object.</p>
<p>This tool adds flexibility to existing functions or classes whose output
is hardwired to stdout.</p>
<p>For example, the output of <a class="reference internal" href="functions.html#help" title="help"><code class="xref py py-func docutils literal notranslate"><span class="pre">help()</span></code></a> normally is sent to <em>sys.stdout</em>.
You can capture that output in a string by redirecting the output to an
<a class="reference internal" href="io.html#io.StringIO" title="io.StringIO"><code class="xref py py-class docutils literal notranslate"><span class="pre">io.StringIO</span></code></a> object:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">f</span> <span class="o">=</span> <span class="n">io</span><span class="o">.</span><span class="n">StringIO</span><span class="p">()</span>
<span class="k">with</span> <span class="n">redirect_stdout</span><span class="p">(</span><span class="n">f</span><span class="p">):</span>
<span class="n">help</span><span class="p">(</span><span class="nb">pow</span><span class="p">)</span>
<span class="n">s</span> <span class="o">=</span> <span class="n">f</span><span class="o">.</span><span class="n">getvalue</span><span class="p">()</span>
</pre></div>
</div>
<p>To send the output of <a class="reference internal" href="functions.html#help" title="help"><code class="xref py py-func docutils literal notranslate"><span class="pre">help()</span></code></a> to a file on disk, redirect the output
to a regular file:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="s1">'help.txt'</span><span class="p">,</span> <span class="s1">'w'</span><span class="p">)</span> <span class="k">as</span> <span class="n">f</span><span class="p">:</span>
<span class="k">with</span> <span class="n">redirect_stdout</span><span class="p">(</span><span class="n">f</span><span class="p">):</span>
<span class="n">help</span><span class="p">(</span><span class="nb">pow</span><span class="p">)</span>
</pre></div>
</div>
<p>To send the output of <a class="reference internal" href="functions.html#help" title="help"><code class="xref py py-func docutils literal notranslate"><span class="pre">help()</span></code></a> to <em>sys.stderr</em>:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">with</span> <span class="n">redirect_stdout</span><span class="p">(</span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">):</span>
<span class="n">help</span><span class="p">(</span><span class="nb">pow</span><span class="p">)</span>
</pre></div>
</div>
<p>Note that the global side effect on <a class="reference internal" href="sys.html#sys.stdout" title="sys.stdout"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.stdout</span></code></a> means that this
context manager is not suitable for use in library code and most threaded
applications. It also has no effect on the output of subprocesses.
However, it is still a useful approach for many utility scripts.</p>
<p>This context manager is <a class="reference internal" href="#reentrant-cms"><span class="std std-ref">reentrant</span></a>.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.4.</span></p>
</div>
</dd></dl>
<dl class="function">
<dt id="contextlib.redirect_stderr">
<code class="sig-prename descclassname">contextlib.</code><code class="sig-name descname">redirect_stderr</code><span class="sig-paren">(</span><em class="sig-param">new_target</em><span class="sig-paren">)</span><a class="headerlink" href="#contextlib.redirect_stderr" title="Permalink to this definition">¶</a></dt>
<dd><p>Similar to <a class="reference internal" href="#contextlib.redirect_stdout" title="contextlib.redirect_stdout"><code class="xref py py-func docutils literal notranslate"><span class="pre">redirect_stdout()</span></code></a> but redirecting
<a class="reference internal" href="sys.html#sys.stderr" title="sys.stderr"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.stderr</span></code></a> to another file or file-like object.</p>
<p>This context manager is <a class="reference internal" href="#reentrant-cms"><span class="std std-ref">reentrant</span></a>.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.5.</span></p>
</div>
</dd></dl>
<dl class="class">
<dt id="contextlib.ContextDecorator">
<em class="property">class </em><code class="sig-prename descclassname">contextlib.</code><code class="sig-name descname">ContextDecorator</code><a class="headerlink" href="#contextlib.ContextDecorator" title="Permalink to this definition">¶</a></dt>
<dd><p>A base class that enables a context manager to also be used as a decorator.</p>
<p>Context managers inheriting from <code class="docutils literal notranslate"><span class="pre">ContextDecorator</span></code> have to implement
<code class="docutils literal notranslate"><span class="pre">__enter__</span></code> and <code class="docutils literal notranslate"><span class="pre">__exit__</span></code> as normal. <code class="docutils literal notranslate"><span class="pre">__exit__</span></code> retains its optional
exception handling even when used as a decorator.</p>
<p><code class="docutils literal notranslate"><span class="pre">ContextDecorator</span></code> is used by <a class="reference internal" href="#contextlib.contextmanager" title="contextlib.contextmanager"><code class="xref py py-func docutils literal notranslate"><span class="pre">contextmanager()</span></code></a>, so you get this
functionality automatically.</p>
<p>Example of <code class="docutils literal notranslate"><span class="pre">ContextDecorator</span></code>:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">ContextDecorator</span>
<span class="k">class</span> <span class="nc">mycontext</span><span class="p">(</span><span class="n">ContextDecorator</span><span class="p">):</span>
<span class="k">def</span> <span class="fm">__enter__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="nb">print</span><span class="p">(</span><span class="s1">'Starting'</span><span class="p">)</span>
<span class="k">return</span> <span class="bp">self</span>
<span class="k">def</span> <span class="fm">__exit__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="o">*</span><span class="n">exc</span><span class="p">):</span>
<span class="nb">print</span><span class="p">(</span><span class="s1">'Finishing'</span><span class="p">)</span>
<span class="k">return</span> <span class="kc">False</span>
<span class="o">>>></span> <span class="nd">@mycontext</span><span class="p">()</span>
<span class="o">...</span> <span class="k">def</span> <span class="nf">function</span><span class="p">():</span>
<span class="o">...</span> <span class="nb">print</span><span class="p">(</span><span class="s1">'The bit in the middle'</span><span class="p">)</span>
<span class="o">...</span>
<span class="o">>>></span> <span class="n">function</span><span class="p">()</span>
<span class="n">Starting</span>
<span class="n">The</span> <span class="n">bit</span> <span class="ow">in</span> <span class="n">the</span> <span class="n">middle</span>
<span class="n">Finishing</span>
<span class="o">>>></span> <span class="k">with</span> <span class="n">mycontext</span><span class="p">():</span>
<span class="o">...</span> <span class="nb">print</span><span class="p">(</span><span class="s1">'The bit in the middle'</span><span class="p">)</span>
<span class="o">...</span>
<span class="n">Starting</span>
<span class="n">The</span> <span class="n">bit</span> <span class="ow">in</span> <span class="n">the</span> <span class="n">middle</span>
<span class="n">Finishing</span>
</pre></div>
</div>
<p>This change is just syntactic sugar for any construct of the following form:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">f</span><span class="p">():</span>
<span class="k">with</span> <span class="n">cm</span><span class="p">():</span>
<span class="c1"># Do stuff</span>
</pre></div>
</div>
<p><code class="docutils literal notranslate"><span class="pre">ContextDecorator</span></code> lets you instead write:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="nd">@cm</span><span class="p">()</span>
<span class="k">def</span> <span class="nf">f</span><span class="p">():</span>
<span class="c1"># Do stuff</span>
</pre></div>
</div>
<p>It makes it clear that the <code class="docutils literal notranslate"><span class="pre">cm</span></code> applies to the whole function, rather than
just a piece of it (and saving an indentation level is nice, too).</p>
<p>Existing context managers that already have a base class can be extended by
using <code class="docutils literal notranslate"><span class="pre">ContextDecorator</span></code> as a mixin class:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">ContextDecorator</span>
<span class="k">class</span> <span class="nc">mycontext</span><span class="p">(</span><span class="n">ContextBaseClass</span><span class="p">,</span> <span class="n">ContextDecorator</span><span class="p">):</span>
<span class="k">def</span> <span class="fm">__enter__</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="k">def</span> <span class="fm">__exit__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="o">*</span><span class="n">exc</span><span class="p">):</span>
<span class="k">return</span> <span class="kc">False</span>
</pre></div>
</div>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>As the decorated function must be able to be called multiple times, the
underlying context manager must support use in multiple <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a>
statements. If this is not the case, then the original construct with the
explicit <code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code> statement inside the function should be used.</p>
</div>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.2.</span></p>
</div>
</dd></dl>
<dl class="class">
<dt id="contextlib.ExitStack">
<em class="property">class </em><code class="sig-prename descclassname">contextlib.</code><code class="sig-name descname">ExitStack</code><a class="headerlink" href="#contextlib.ExitStack" title="Permalink to this definition">¶</a></dt>
<dd><p>A context manager that is designed to make it easy to programmatically
combine other context managers and cleanup functions, especially those
that are optional or otherwise driven by input data.</p>
<p>For example, a set of files may easily be handled in a single with
statement as follows:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">with</span> <span class="n">ExitStack</span><span class="p">()</span> <span class="k">as</span> <span class="n">stack</span><span class="p">:</span>
<span class="n">files</span> <span class="o">=</span> <span class="p">[</span><span class="n">stack</span><span class="o">.</span><span class="n">enter_context</span><span class="p">(</span><span class="nb">open</span><span class="p">(</span><span class="n">fname</span><span class="p">))</span> <span class="k">for</span> <span class="n">fname</span> <span class="ow">in</span> <span class="n">filenames</span><span class="p">]</span>
<span class="c1"># All opened files will automatically be closed at the end of</span>
<span class="c1"># the with statement, even if attempts to open files later</span>
<span class="c1"># in the list raise an exception</span>
</pre></div>
</div>
<p>Each instance maintains a stack of registered callbacks that are called in
reverse order when the instance is closed (either explicitly or implicitly
at the end of a <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement). Note that callbacks are <em>not</em>
invoked implicitly when the context stack instance is garbage collected.</p>
<p>This stack model is used so that context managers that acquire their
resources in their <code class="docutils literal notranslate"><span class="pre">__init__</span></code> method (such as file objects) can be
handled correctly.</p>
<p>Since registered callbacks are invoked in the reverse order of
registration, this ends up behaving as if multiple nested <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a>
statements had been used with the registered set of callbacks. This even
extends to exception handling - if an inner callback suppresses or replaces
an exception, then outer callbacks will be passed arguments based on that
updated state.</p>
<p>This is a relatively low level API that takes care of the details of
correctly unwinding the stack of exit callbacks. It provides a suitable
foundation for higher level context managers that manipulate the exit
stack in application specific ways.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.3.</span></p>
</div>
<dl class="method">
<dt id="contextlib.ExitStack.enter_context">
<code class="sig-name descname">enter_context</code><span class="sig-paren">(</span><em class="sig-param">cm</em><span class="sig-paren">)</span><a class="headerlink" href="#contextlib.ExitStack.enter_context" title="Permalink to this definition">¶</a></dt>
<dd><p>Enters a new context manager and adds its <a class="reference internal" href="../reference/datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__exit__()</span></code></a> method to
the callback stack. The return value is the result of the context
manager’s own <a class="reference internal" href="../reference/datamodel.html#object.__enter__" title="object.__enter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__enter__()</span></code></a> method.</p>
<p>These context managers may suppress exceptions just as they normally
would if used directly as part of a <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement.</p>
</dd></dl>
<dl class="method">
<dt id="contextlib.ExitStack.push">
<code class="sig-name descname">push</code><span class="sig-paren">(</span><em class="sig-param">exit</em><span class="sig-paren">)</span><a class="headerlink" href="#contextlib.ExitStack.push" title="Permalink to this definition">¶</a></dt>
<dd><p>Adds a context manager’s <a class="reference internal" href="../reference/datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__exit__()</span></code></a> method to the callback stack.</p>
<p>As <code class="docutils literal notranslate"><span class="pre">__enter__</span></code> is <em>not</em> invoked, this method can be used to cover
part of an <a class="reference internal" href="../reference/datamodel.html#object.__enter__" title="object.__enter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__enter__()</span></code></a> implementation with a context manager’s own
<a class="reference internal" href="../reference/datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__exit__()</span></code></a> method.</p>
<p>If passed an object that is not a context manager, this method assumes
it is a callback with the same signature as a context manager’s
<a class="reference internal" href="../reference/datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__exit__()</span></code></a> method and adds it directly to the callback stack.</p>
<p>By returning true values, these callbacks can suppress exceptions the
same way context manager <a class="reference internal" href="../reference/datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__exit__()</span></code></a> methods can.</p>
<p>The passed in object is returned from the function, allowing this
method to be used as a function decorator.</p>
</dd></dl>
<dl class="method">
<dt id="contextlib.ExitStack.callback">
<code class="sig-name descname">callback</code><span class="sig-paren">(</span><em class="sig-param">callback</em>, <em class="sig-param">*args</em>, <em class="sig-param">**kwds</em><span class="sig-paren">)</span><a class="headerlink" href="#contextlib.ExitStack.callback" title="Permalink to this definition">¶</a></dt>
<dd><p>Accepts an arbitrary callback function and arguments and adds it to
the callback stack.</p>
<p>Unlike the other methods, callbacks added this way cannot suppress
exceptions (as they are never passed the exception details).</p>
<p>The passed in callback is returned from the function, allowing this
method to be used as a function decorator.</p>
</dd></dl>
<dl class="method">
<dt id="contextlib.ExitStack.pop_all">
<code class="sig-name descname">pop_all</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#contextlib.ExitStack.pop_all" title="Permalink to this definition">¶</a></dt>
<dd><p>Transfers the callback stack to a fresh <a class="reference internal" href="#contextlib.ExitStack" title="contextlib.ExitStack"><code class="xref py py-class docutils literal notranslate"><span class="pre">ExitStack</span></code></a> instance
and returns it. No callbacks are invoked by this operation - instead,
they will now be invoked when the new stack is closed (either
explicitly or implicitly at the end of a <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement).</p>
<p>For example, a group of files can be opened as an “all or nothing”
operation as follows:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">with</span> <span class="n">ExitStack</span><span class="p">()</span> <span class="k">as</span> <span class="n">stack</span><span class="p">:</span>
<span class="n">files</span> <span class="o">=</span> <span class="p">[</span><span class="n">stack</span><span class="o">.</span><span class="n">enter_context</span><span class="p">(</span><span class="nb">open</span><span class="p">(</span><span class="n">fname</span><span class="p">))</span> <span class="k">for</span> <span class="n">fname</span> <span class="ow">in</span> <span class="n">filenames</span><span class="p">]</span>
<span class="c1"># Hold onto the close method, but don't call it yet.</span>
<span class="n">close_files</span> <span class="o">=</span> <span class="n">stack</span><span class="o">.</span><span class="n">pop_all</span><span class="p">()</span><span class="o">.</span><span class="n">close</span>
<span class="c1"># If opening any file fails, all previously opened files will be</span>
<span class="c1"># closed automatically. If all files are opened successfully,</span>
<span class="c1"># they will remain open even after the with statement ends.</span>
<span class="c1"># close_files() can then be invoked explicitly to close them all.</span>
</pre></div>
</div>
</dd></dl>
<dl class="method">
<dt id="contextlib.ExitStack.close">
<code class="sig-name descname">close</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#contextlib.ExitStack.close" title="Permalink to this definition">¶</a></dt>
<dd><p>Immediately unwinds the callback stack, invoking callbacks in the
reverse order of registration. For any context managers and exit
callbacks registered, the arguments passed in will indicate that no
exception occurred.</p>
</dd></dl>
</dd></dl>
<dl class="class">
<dt id="contextlib.AsyncExitStack">
<em class="property">class </em><code class="sig-prename descclassname">contextlib.</code><code class="sig-name descname">AsyncExitStack</code><a class="headerlink" href="#contextlib.AsyncExitStack" title="Permalink to this definition">¶</a></dt>
<dd><p>An <a class="reference internal" href="../reference/datamodel.html#async-context-managers"><span class="std std-ref">asynchronous context manager</span></a>, similar
to <a class="reference internal" href="#contextlib.ExitStack" title="contextlib.ExitStack"><code class="xref py py-class docutils literal notranslate"><span class="pre">ExitStack</span></code></a>, that supports combining both synchronous and
asynchronous context managers, as well as having coroutines for
cleanup logic.</p>
<p>The <code class="xref py py-meth docutils literal notranslate"><span class="pre">close()</span></code> method is not implemented, <a class="reference internal" href="#contextlib.AsyncExitStack.aclose" title="contextlib.AsyncExitStack.aclose"><code class="xref py py-meth docutils literal notranslate"><span class="pre">aclose()</span></code></a> must be used
instead.</p>
<dl class="method">
<dt id="contextlib.AsyncExitStack.enter_async_context">
<code class="sig-name descname">enter_async_context</code><span class="sig-paren">(</span><em class="sig-param">cm</em><span class="sig-paren">)</span><a class="headerlink" href="#contextlib.AsyncExitStack.enter_async_context" title="Permalink to this definition">¶</a></dt>
<dd><p>Similar to <code class="xref py py-meth docutils literal notranslate"><span class="pre">enter_context()</span></code> but expects an asynchronous context
manager.</p>
</dd></dl>
<dl class="method">
<dt id="contextlib.AsyncExitStack.push_async_exit">
<code class="sig-name descname">push_async_exit</code><span class="sig-paren">(</span><em class="sig-param">exit</em><span class="sig-paren">)</span><a class="headerlink" href="#contextlib.AsyncExitStack.push_async_exit" title="Permalink to this definition">¶</a></dt>
<dd><p>Similar to <code class="xref py py-meth docutils literal notranslate"><span class="pre">push()</span></code> but expects either an asynchronous context manager
or a coroutine function.</p>
</dd></dl>
<dl class="method">
<dt id="contextlib.AsyncExitStack.push_async_callback">
<code class="sig-name descname">push_async_callback</code><span class="sig-paren">(</span><em class="sig-param">callback</em>, <em class="sig-param">*args</em>, <em class="sig-param">**kwds</em><span class="sig-paren">)</span><a class="headerlink" href="#contextlib.AsyncExitStack.push_async_callback" title="Permalink to this definition">¶</a></dt>
<dd><p>Similar to <code class="xref py py-meth docutils literal notranslate"><span class="pre">callback()</span></code> but expects a coroutine function.</p>
</dd></dl>
<dl class="method">
<dt id="contextlib.AsyncExitStack.aclose">
<code class="sig-name descname">aclose</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#contextlib.AsyncExitStack.aclose" title="Permalink to this definition">¶</a></dt>
<dd><p>Similar to <code class="xref py py-meth docutils literal notranslate"><span class="pre">close()</span></code> but properly handles awaitables.</p>
</dd></dl>
<p>Continuing the example for <a class="reference internal" href="#contextlib.asynccontextmanager" title="contextlib.asynccontextmanager"><code class="xref py py-func docutils literal notranslate"><span class="pre">asynccontextmanager()</span></code></a>:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">async</span> <span class="k">with</span> <span class="n">AsyncExitStack</span><span class="p">()</span> <span class="k">as</span> <span class="n">stack</span><span class="p">:</span>
<span class="n">connections</span> <span class="o">=</span> <span class="p">[</span><span class="k">await</span> <span class="n">stack</span><span class="o">.</span><span class="n">enter_async_context</span><span class="p">(</span><span class="n">get_connection</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="c1"># All opened connections will automatically be released at the end of</span>
<span class="c1"># the async with statement, even if attempts to open a connection</span>
<span class="c1"># later in the list raise an exception.</span>
</pre></div>
</div>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.7.</span></p>
</div>
</dd></dl>
</div>
<div class="section" id="examples-and-recipes">
<h2>Examples and Recipes<a class="headerlink" href="#examples-and-recipes" title="Permalink to this headline">¶</a></h2>
<p>This section describes some examples and recipes for making effective use of
the tools provided by <a class="reference internal" href="#module-contextlib" title="contextlib: Utilities for with-statement contexts."><code class="xref py py-mod docutils literal notranslate"><span class="pre">contextlib</span></code></a>.</p>
<div class="section" id="supporting-a-variable-number-of-context-managers">
<h3>Supporting a variable number of context managers<a class="headerlink" href="#supporting-a-variable-number-of-context-managers" title="Permalink to this headline">¶</a></h3>
<p>The primary use case for <a class="reference internal" href="#contextlib.ExitStack" title="contextlib.ExitStack"><code class="xref py py-class docutils literal notranslate"><span class="pre">ExitStack</span></code></a> is the one given in the class
documentation: supporting a variable number of context managers and other
cleanup operations in a single <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement. The variability
may come from the number of context managers needed being driven by user
input (such as opening a user specified collection of files), or from
some of the context managers being optional:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">with</span> <span class="n">ExitStack</span><span class="p">()</span> <span class="k">as</span> <span class="n">stack</span><span class="p">:</span>
<span class="k">for</span> <span class="n">resource</span> <span class="ow">in</span> <span class="n">resources</span><span class="p">:</span>
<span class="n">stack</span><span class="o">.</span><span class="n">enter_context</span><span class="p">(</span><span class="n">resource</span><span class="p">)</span>
<span class="k">if</span> <span class="n">need_special_resource</span><span class="p">():</span>
<span class="n">special</span> <span class="o">=</span> <span class="n">acquire_special_resource</span><span class="p">()</span>
<span class="n">stack</span><span class="o">.</span><span class="n">callback</span><span class="p">(</span><span class="n">release_special_resource</span><span class="p">,</span> <span class="n">special</span><span class="p">)</span>
<span class="c1"># Perform operations that use the acquired resources</span>
</pre></div>
</div>
<p>As shown, <a class="reference internal" href="#contextlib.ExitStack" title="contextlib.ExitStack"><code class="xref py py-class docutils literal notranslate"><span class="pre">ExitStack</span></code></a> also makes it quite easy to use <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a>
statements to manage arbitrary resources that don’t natively support the
context management protocol.</p>
</div>
<div class="section" id="catching-exceptions-from-enter-methods">
<h3>Catching exceptions from <code class="docutils literal notranslate"><span class="pre">__enter__</span></code> methods<a class="headerlink" href="#catching-exceptions-from-enter-methods" title="Permalink to this headline">¶</a></h3>
<p>It is occasionally desirable to catch exceptions from an <code class="docutils literal notranslate"><span class="pre">__enter__</span></code>
method implementation, <em>without</em> inadvertently catching exceptions from
the <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement body or the context manager’s <code class="docutils literal notranslate"><span class="pre">__exit__</span></code>
method. By using <a class="reference internal" href="#contextlib.ExitStack" title="contextlib.ExitStack"><code class="xref py py-class docutils literal notranslate"><span class="pre">ExitStack</span></code></a> the steps in the context management
protocol can be separated slightly in order to allow this:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">stack</span> <span class="o">=</span> <span class="n">ExitStack</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="n">stack</span><span class="o">.</span><span class="n">enter_context</span><span class="p">(</span><span class="n">cm</span><span class="p">)</span>
<span class="k">except</span> <span class="ne">Exception</span><span class="p">:</span>
<span class="c1"># handle __enter__ exception</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">with</span> <span class="n">stack</span><span class="p">:</span>
<span class="c1"># Handle normal case</span>
</pre></div>
</div>
<p>Actually needing to do this is likely to indicate that the underlying API
should be providing a direct resource management interface for use with
<a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a>/<a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a>/<a class="reference internal" href="../reference/compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> statements, but not
all APIs are well designed in that regard. When a context manager is the
only resource management API provided, then <a class="reference internal" href="#contextlib.ExitStack" title="contextlib.ExitStack"><code class="xref py py-class docutils literal notranslate"><span class="pre">ExitStack</span></code></a> can make it
easier to handle various situations that can’t be handled directly in a
<a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement.</p>
</div>
<div class="section" id="cleaning-up-in-an-enter-implementation">
<h3>Cleaning up in an <code class="docutils literal notranslate"><span class="pre">__enter__</span></code> implementation<a class="headerlink" href="#cleaning-up-in-an-enter-implementation" title="Permalink to this headline">¶</a></h3>
<p>As noted in the documentation of <a class="reference internal" href="#contextlib.ExitStack.push" title="contextlib.ExitStack.push"><code class="xref py py-meth docutils literal notranslate"><span class="pre">ExitStack.push()</span></code></a>, this
method can be useful in cleaning up an already allocated resource if later
steps in the <a class="reference internal" href="../reference/datamodel.html#object.__enter__" title="object.__enter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__enter__()</span></code></a> implementation fail.</p>
<p>Here’s an example of doing this for a context manager that accepts resource
acquisition and release functions, along with an optional validation function,
and maps them to the context management protocol:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">contextmanager</span><span class="p">,</span> <span class="n">AbstractContextManager</span><span class="p">,</span> <span class="n">ExitStack</span>
<span class="k">class</span> <span class="nc">ResourceManager</span><span class="p">(</span><span class="n">AbstractContextManager</span><span class="p">):</span>
<span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">acquire_resource</span><span class="p">,</span> <span class="n">release_resource</span><span class="p">,</span> <span class="n">check_resource_ok</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">acquire_resource</span> <span class="o">=</span> <span class="n">acquire_resource</span>
<span class="bp">self</span><span class="o">.</span><span class="n">release_resource</span> <span class="o">=</span> <span class="n">release_resource</span>
<span class="k">if</span> <span class="n">check_resource_ok</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
<span class="k">def</span> <span class="nf">check_resource_ok</span><span class="p">(</span><span class="n">resource</span><span class="p">):</span>
<span class="k">return</span> <span class="kc">True</span>
<span class="bp">self</span><span class="o">.</span><span class="n">check_resource_ok</span> <span class="o">=</span> <span class="n">check_resource_ok</span>
<span class="nd">@contextmanager</span>
<span class="k">def</span> <span class="nf">_cleanup_on_error</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">with</span> <span class="n">ExitStack</span><span class="p">()</span> <span class="k">as</span> <span class="n">stack</span><span class="p">:</span>
<span class="n">stack</span><span class="o">.</span><span class="n">push</span><span class="p">(</span><span class="bp">self</span><span class="p">)</span>
<span class="k">yield</span>
<span class="c1"># The validation check passed and didn't raise an exception</span>
<span class="c1"># Accordingly, we want to keep the resource, and pass it</span>
<span class="c1"># back to our caller</span>
<span class="n">stack</span><span class="o">.</span><span class="n">pop_all</span><span class="p">()</span>
<span class="k">def</span> <span class="fm">__enter__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="n">resource</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">acquire_resource</span><span class="p">()</span>
<span class="k">with</span> <span class="bp">self</span><span class="o">.</span><span class="n">_cleanup_on_error</span><span class="p">():</span>
<span class="k">if</span> <span class="ow">not</span> <span class="bp">self</span><span class="o">.</span><span class="n">check_resource_ok</span><span class="p">(</span><span class="n">resource</span><span class="p">):</span>
<span class="n">msg</span> <span class="o">=</span> <span class="s2">"Failed validation for </span><span class="si">{!r}</span><span class="s2">"</span>
<span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="n">msg</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">resource</span><span class="p">))</span>
<span class="k">return</span> <span class="n">resource</span>
<span class="k">def</span> <span class="fm">__exit__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="o">*</span><span class="n">exc_details</span><span class="p">):</span>
<span class="c1"># We don't need to duplicate any of our resource release logic</span>
<span class="bp">self</span><span class="o">.</span><span class="n">release_resource</span><span class="p">()</span>
</pre></div>
</div>
</div>
<div class="section" id="replacing-any-use-of-try-finally-and-flag-variables">
<h3>Replacing any use of <code class="docutils literal notranslate"><span class="pre">try-finally</span></code> and flag variables<a class="headerlink" href="#replacing-any-use-of-try-finally-and-flag-variables" title="Permalink to this headline">¶</a></h3>
<p>A pattern you will sometimes see is a <code class="docutils literal notranslate"><span class="pre">try-finally</span></code> statement with a flag
variable to indicate whether or not the body of the <code class="docutils literal notranslate"><span class="pre">finally</span></code> clause should
be executed. In its simplest form (that can’t already be handled just by
using an <code class="docutils literal notranslate"><span class="pre">except</span></code> clause instead), it looks something like this:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">cleanup_needed</span> <span class="o">=</span> <span class="kc">True</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">result</span> <span class="o">=</span> <span class="n">perform_operation</span><span class="p">()</span>
<span class="k">if</span> <span class="n">result</span><span class="p">:</span>
<span class="n">cleanup_needed</span> <span class="o">=</span> <span class="kc">False</span>
<span class="k">finally</span><span class="p">:</span>
<span class="k">if</span> <span class="n">cleanup_needed</span><span class="p">:</span>
<span class="n">cleanup_resources</span><span class="p">()</span>
</pre></div>
</div>
<p>As with any <code class="docutils literal notranslate"><span class="pre">try</span></code> statement based code, this can cause problems for
development and review, because the setup code and the cleanup code can end
up being separated by arbitrarily long sections of code.</p>
<p><a class="reference internal" href="#contextlib.ExitStack" title="contextlib.ExitStack"><code class="xref py py-class docutils literal notranslate"><span class="pre">ExitStack</span></code></a> makes it possible to instead register a callback for
execution at the end of a <code class="docutils literal notranslate"><span class="pre">with</span></code> statement, and then later decide to skip
executing that callback:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">ExitStack</span>
<span class="k">with</span> <span class="n">ExitStack</span><span class="p">()</span> <span class="k">as</span> <span class="n">stack</span><span class="p">:</span>
<span class="n">stack</span><span class="o">.</span><span class="n">callback</span><span class="p">(</span><span class="n">cleanup_resources</span><span class="p">)</span>
<span class="n">result</span> <span class="o">=</span> <span class="n">perform_operation</span><span class="p">()</span>
<span class="k">if</span> <span class="n">result</span><span class="p">:</span>
<span class="n">stack</span><span class="o">.</span><span class="n">pop_all</span><span class="p">()</span>
</pre></div>
</div>
<p>This allows the intended cleanup up behaviour to be made explicit up front,
rather than requiring a separate flag variable.</p>
<p>If a particular application uses this pattern a lot, it can be simplified
even further by means of a small helper class:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">ExitStack</span>
<span class="k">class</span> <span class="nc">Callback</span><span class="p">(</span><span class="n">ExitStack</span><span class="p">):</span>
<span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">callback</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">kwds</span><span class="p">):</span>
<span class="nb">super</span><span class="p">(</span><span class="n">Callback</span><span class="p">,</span> <span class="bp">self</span><span class="p">)</span><span class="o">.</span><span class="fm">__init__</span><span class="p">()</span>
<span class="bp">self</span><span class="o">.</span><span class="n">callback</span><span class="p">(</span><span class="n">callback</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">kwds</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">cancel</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">pop_all</span><span class="p">()</span>
<span class="k">with</span> <span class="n">Callback</span><span class="p">(</span><span class="n">cleanup_resources</span><span class="p">)</span> <span class="k">as</span> <span class="n">cb</span><span class="p">:</span>
<span class="n">result</span> <span class="o">=</span> <span class="n">perform_operation</span><span class="p">()</span>
<span class="k">if</span> <span class="n">result</span><span class="p">:</span>
<span class="n">cb</span><span class="o">.</span><span class="n">cancel</span><span class="p">()</span>
</pre></div>
</div>
<p>If the resource cleanup isn’t already neatly bundled into a standalone
function, then it is still possible to use the decorator form of
<a class="reference internal" href="#contextlib.ExitStack.callback" title="contextlib.ExitStack.callback"><code class="xref py py-meth docutils literal notranslate"><span class="pre">ExitStack.callback()</span></code></a> to declare the resource cleanup in
advance:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">ExitStack</span>
<span class="k">with</span> <span class="n">ExitStack</span><span class="p">()</span> <span class="k">as</span> <span class="n">stack</span><span class="p">:</span>
<span class="nd">@stack</span><span class="o">.</span><span class="n">callback</span>
<span class="k">def</span> <span class="nf">cleanup_resources</span><span class="p">():</span>
<span class="o">...</span>
<span class="n">result</span> <span class="o">=</span> <span class="n">perform_operation</span><span class="p">()</span>
<span class="k">if</span> <span class="n">result</span><span class="p">:</span>
<span class="n">stack</span><span class="o">.</span><span class="n">pop_all</span><span class="p">()</span>
</pre></div>
</div>
<p>Due to the way the decorator protocol works, a callback function
declared this way cannot take any parameters. Instead, any resources to
be released must be accessed as closure variables.</p>
</div>
<div class="section" id="using-a-context-manager-as-a-function-decorator">
<h3>Using a context manager as a function decorator<a class="headerlink" href="#using-a-context-manager-as-a-function-decorator" title="Permalink to this headline">¶</a></h3>
<p><a class="reference internal" href="#contextlib.ContextDecorator" title="contextlib.ContextDecorator"><code class="xref py py-class docutils literal notranslate"><span class="pre">ContextDecorator</span></code></a> makes it possible to use a context manager in
both an ordinary <code class="docutils literal notranslate"><span class="pre">with</span></code> statement and also as a function decorator.</p>
<p>For example, it is sometimes useful to wrap functions or groups of statements
with a logger that can track the time of entry and time of exit. Rather than
writing both a function decorator and a context manager for the task,
inheriting from <a class="reference internal" href="#contextlib.ContextDecorator" title="contextlib.ContextDecorator"><code class="xref py py-class docutils literal notranslate"><span class="pre">ContextDecorator</span></code></a> provides both capabilities in a
single definition:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">ContextDecorator</span>
<span class="kn">import</span> <span class="nn">logging</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="k">class</span> <span class="nc">track_entry_and_exit</span><span class="p">(</span><span class="n">ContextDecorator</span><span class="p">):</span>
<span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">name</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="n">name</span>
<span class="k">def</span> <span class="fm">__enter__</span><span class="p">(</span><span class="bp">self</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">'Entering: </span><span class="si">%s</span><span class="s1">'</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">name</span><span class="p">)</span>
<span class="k">def</span> <span class="fm">__exit__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">exc_type</span><span class="p">,</span> <span class="n">exc</span><span class="p">,</span> <span class="n">exc_tb</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">'Exiting: </span><span class="si">%s</span><span class="s1">'</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">name</span><span class="p">)</span>
</pre></div>
</div>
<p>Instances of this class can be used as both a context manager:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">with</span> <span class="n">track_entry_and_exit</span><span class="p">(</span><span class="s1">'widget loader'</span><span class="p">):</span>
<span class="nb">print</span><span class="p">(</span><span class="s1">'Some time consuming activity goes here'</span><span class="p">)</span>
<span class="n">load_widget</span><span class="p">()</span>
</pre></div>
</div>
<p>And also as a function decorator:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="nd">@track_entry_and_exit</span><span class="p">(</span><span class="s1">'widget loader'</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">activity</span><span class="p">():</span>
<span class="nb">print</span><span class="p">(</span><span class="s1">'Some time consuming activity goes here'</span><span class="p">)</span>
<span class="n">load_widget</span><span class="p">()</span>
</pre></div>
</div>
<p>Note that there is one additional limitation when using context managers
as function decorators: there’s no way to access the return value of
<a class="reference internal" href="../reference/datamodel.html#object.__enter__" title="object.__enter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__enter__()</span></code></a>. If that value is needed, then it is still necessary to use
an explicit <code class="docutils literal notranslate"><span class="pre">with</span></code> statement.</p>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<dl class="simple">
<dt><span class="target" id="index-0"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0343"><strong>PEP 343</strong></a> - The “with” statement</dt><dd><p>The specification, background, and examples for the Python <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a>
statement.</p>
</dd>
</dl>
</div>
</div>
</div>
<div class="section" id="single-use-reusable-and-reentrant-context-managers">
<span id="single-use-reusable-and-reentrant-cms"></span><h2>Single use, reusable and reentrant context managers<a class="headerlink" href="#single-use-reusable-and-reentrant-context-managers" title="Permalink to this headline">¶</a></h2>
<p>Most context managers are written in a way that means they can only be
used effectively in a <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement once. These single use
context managers must be created afresh each time they’re used -
attempting to use them a second time will trigger an exception or
otherwise not work correctly.</p>
<p>This common limitation means that it is generally advisable to create
context managers directly in the header of the <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement
where they are used (as shown in all of the usage examples above).</p>
<p>Files are an example of effectively single use context managers, since
the first <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement will close the file, preventing any
further IO operations using that file object.</p>
<p>Context managers created using <a class="reference internal" href="#contextlib.contextmanager" title="contextlib.contextmanager"><code class="xref py py-func docutils literal notranslate"><span class="pre">contextmanager()</span></code></a> are also single use
context managers, and will complain about the underlying generator failing
to yield if an attempt is made to use them a second time:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">contextmanager</span>
<span class="gp">>>> </span><span class="nd">@contextmanager</span>
<span class="gp">... </span><span class="k">def</span> <span class="nf">singleuse</span><span class="p">():</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"Before"</span><span class="p">)</span>
<span class="gp">... </span> <span class="k">yield</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"After"</span><span class="p">)</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="n">cm</span> <span class="o">=</span> <span class="n">singleuse</span><span class="p">()</span>
<span class="gp">>>> </span><span class="k">with</span> <span class="n">cm</span><span class="p">:</span>
<span class="gp">... </span> <span class="k">pass</span>
<span class="gp">...</span>
<span class="go">Before</span>
<span class="go">After</span>
<span class="gp">>>> </span><span class="k">with</span> <span class="n">cm</span><span class="p">:</span>
<span class="gp">... </span> <span class="k">pass</span>
<span class="gp">...</span>
<span class="gt">Traceback (most recent call last):</span>
<span class="o">...</span>
<span class="gr">RuntimeError</span>: <span class="n">generator didn't yield</span>
</pre></div>
</div>
<div class="section" id="reentrant-context-managers">
<span id="reentrant-cms"></span><h3>Reentrant context managers<a class="headerlink" href="#reentrant-context-managers" title="Permalink to this headline">¶</a></h3>
<p>More sophisticated context managers may be “reentrant”. These context
managers can not only be used in multiple <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statements,
but may also be used <em>inside</em> a <code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code> statement that is already
using the same context manager.</p>
<p><a class="reference internal" href="threading.html#threading.RLock" title="threading.RLock"><code class="xref py py-class docutils literal notranslate"><span class="pre">threading.RLock</span></code></a> is an example of a reentrant context manager, as are
<a class="reference internal" href="#contextlib.suppress" title="contextlib.suppress"><code class="xref py py-func docutils literal notranslate"><span class="pre">suppress()</span></code></a> and <a class="reference internal" href="#contextlib.redirect_stdout" title="contextlib.redirect_stdout"><code class="xref py py-func docutils literal notranslate"><span class="pre">redirect_stdout()</span></code></a>. Here’s a very simple example of
reentrant use:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">redirect_stdout</span>
<span class="gp">>>> </span><span class="kn">from</span> <span class="nn">io</span> <span class="kn">import</span> <span class="n">StringIO</span>
<span class="gp">>>> </span><span class="n">stream</span> <span class="o">=</span> <span class="n">StringIO</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">write_to_stream</span> <span class="o">=</span> <span class="n">redirect_stdout</span><span class="p">(</span><span class="n">stream</span><span class="p">)</span>
<span class="gp">>>> </span><span class="k">with</span> <span class="n">write_to_stream</span><span class="p">:</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"This is written to the stream rather than stdout"</span><span class="p">)</span>
<span class="gp">... </span> <span class="k">with</span> <span class="n">write_to_stream</span><span class="p">:</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"This is also written to the stream"</span><span class="p">)</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="s2">"This is written directly to stdout"</span><span class="p">)</span>
<span class="go">This is written directly to stdout</span>
<span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="n">stream</span><span class="o">.</span><span class="n">getvalue</span><span class="p">())</span>
<span class="go">This is written to the stream rather than stdout</span>
<span class="go">This is also written to the stream</span>
</pre></div>
</div>
<p>Real world examples of reentrancy are more likely to involve multiple
functions calling each other and hence be far more complicated than this
example.</p>
<p>Note also that being reentrant is <em>not</em> the same thing as being thread safe.
<a class="reference internal" href="#contextlib.redirect_stdout" title="contextlib.redirect_stdout"><code class="xref py py-func docutils literal notranslate"><span class="pre">redirect_stdout()</span></code></a>, for example, is definitely not thread safe, as it
makes a global modification to the system state by binding <a class="reference internal" href="sys.html#sys.stdout" title="sys.stdout"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.stdout</span></code></a>
to a different stream.</p>
</div>
<div class="section" id="reusable-context-managers">
<span id="reusable-cms"></span><h3>Reusable context managers<a class="headerlink" href="#reusable-context-managers" title="Permalink to this headline">¶</a></h3>
<p>Distinct from both single use and reentrant context managers are “reusable”
context managers (or, to be completely explicit, “reusable, but not
reentrant” context managers, since reentrant context managers are also
reusable). These context managers support being used multiple times, but
will fail (or otherwise not work correctly) if the specific context manager
instance has already been used in a containing with statement.</p>
<p><a class="reference internal" href="threading.html#threading.Lock" title="threading.Lock"><code class="xref py py-class docutils literal notranslate"><span class="pre">threading.Lock</span></code></a> is an example of a reusable, but not reentrant,
context manager (for a reentrant lock, it is necessary to use
<a class="reference internal" href="threading.html#threading.RLock" title="threading.RLock"><code class="xref py py-class docutils literal notranslate"><span class="pre">threading.RLock</span></code></a> instead).</p>
<p>Another example of a reusable, but not reentrant, context manager is
<a class="reference internal" href="#contextlib.ExitStack" title="contextlib.ExitStack"><code class="xref py py-class docutils literal notranslate"><span class="pre">ExitStack</span></code></a>, as it invokes <em>all</em> currently registered callbacks
when leaving any with statement, regardless of where those callbacks
were added:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">ExitStack</span>
<span class="gp">>>> </span><span class="n">stack</span> <span class="o">=</span> <span class="n">ExitStack</span><span class="p">()</span>
<span class="gp">>>> </span><span class="k">with</span> <span class="n">stack</span><span class="p">:</span>
<span class="gp">... </span> <span class="n">stack</span><span class="o">.</span><span class="n">callback</span><span class="p">(</span><span class="nb">print</span><span class="p">,</span> <span class="s2">"Callback: from first context"</span><span class="p">)</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"Leaving first context"</span><span class="p">)</span>
<span class="gp">...</span>
<span class="go">Leaving first context</span>
<span class="go">Callback: from first context</span>
<span class="gp">>>> </span><span class="k">with</span> <span class="n">stack</span><span class="p">:</span>
<span class="gp">... </span> <span class="n">stack</span><span class="o">.</span><span class="n">callback</span><span class="p">(</span><span class="nb">print</span><span class="p">,</span> <span class="s2">"Callback: from second context"</span><span class="p">)</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"Leaving second context"</span><span class="p">)</span>
<span class="gp">...</span>
<span class="go">Leaving second context</span>
<span class="go">Callback: from second context</span>
<span class="gp">>>> </span><span class="k">with</span> <span class="n">stack</span><span class="p">:</span>
<span class="gp">... </span> <span class="n">stack</span><span class="o">.</span><span class="n">callback</span><span class="p">(</span><span class="nb">print</span><span class="p">,</span> <span class="s2">"Callback: from outer context"</span><span class="p">)</span>
<span class="gp">... </span> <span class="k">with</span> <span class="n">stack</span><span class="p">:</span>
<span class="gp">... </span> <span class="n">stack</span><span class="o">.</span><span class="n">callback</span><span class="p">(</span><span class="nb">print</span><span class="p">,</span> <span class="s2">"Callback: from inner context"</span><span class="p">)</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"Leaving inner context"</span><span class="p">)</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"Leaving outer context"</span><span class="p">)</span>
<span class="gp">...</span>
<span class="go">Leaving inner context</span>
<span class="go">Callback: from inner context</span>
<span class="go">Callback: from outer context</span>
<span class="go">Leaving outer context</span>
</pre></div>
</div>
<p>As the output from the example shows, reusing a single stack object across
multiple with statements works correctly, but attempting to nest them
will cause the stack to be cleared at the end of the innermost with
statement, which is unlikely to be desirable behaviour.</p>
<p>Using separate <a class="reference internal" href="#contextlib.ExitStack" title="contextlib.ExitStack"><code class="xref py py-class docutils literal notranslate"><span class="pre">ExitStack</span></code></a> instances instead of reusing a single
instance avoids that problem:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">ExitStack</span>
<span class="gp">>>> </span><span class="k">with</span> <span class="n">ExitStack</span><span class="p">()</span> <span class="k">as</span> <span class="n">outer_stack</span><span class="p">:</span>
<span class="gp">... </span> <span class="n">outer_stack</span><span class="o">.</span><span class="n">callback</span><span class="p">(</span><span class="nb">print</span><span class="p">,</span> <span class="s2">"Callback: from outer context"</span><span class="p">)</span>
<span class="gp">... </span> <span class="k">with</span> <span class="n">ExitStack</span><span class="p">()</span> <span class="k">as</span> <span class="n">inner_stack</span><span class="p">:</span>
<span class="gp">... </span> <span class="n">inner_stack</span><span class="o">.</span><span class="n">callback</span><span class="p">(</span><span class="nb">print</span><span class="p">,</span> <span class="s2">"Callback: from inner context"</span><span class="p">)</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"Leaving inner context"</span><span class="p">)</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"Leaving outer context"</span><span class="p">)</span>
<span class="gp">...</span>
<span class="go">Leaving inner context</span>
<span class="go">Callback: from inner context</span>
<span class="go">Leaving outer context</span>
<span class="go">Callback: from outer context</span>
</pre></div>
</div>
</div>
</div>
</div>
</div>
</div>
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<h3><a href="../contents.html">Table of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#"><code class="xref py py-mod docutils literal notranslate"><span class="pre">contextlib</span></code> — Utilities for <code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code>-statement contexts</a><ul>
<li><a class="reference internal" href="#utilities">Utilities</a></li>
<li><a class="reference internal" href="#examples-and-recipes">Examples and Recipes</a><ul>
<li><a class="reference internal" href="#supporting-a-variable-number-of-context-managers">Supporting a variable number of context managers</a></li>
<li><a class="reference internal" href="#catching-exceptions-from-enter-methods">Catching exceptions from <code class="docutils literal notranslate"><span class="pre">__enter__</span></code> methods</a></li>
<li><a class="reference internal" href="#cleaning-up-in-an-enter-implementation">Cleaning up in an <code class="docutils literal notranslate"><span class="pre">__enter__</span></code> implementation</a></li>
<li><a class="reference internal" href="#replacing-any-use-of-try-finally-and-flag-variables">Replacing any use of <code class="docutils literal notranslate"><span class="pre">try-finally</span></code> and flag variables</a></li>
<li><a class="reference internal" href="#using-a-context-manager-as-a-function-decorator">Using a context manager as a function decorator</a></li>
</ul>
</li>
<li><a class="reference internal" href="#single-use-reusable-and-reentrant-context-managers">Single use, reusable and reentrant context managers</a><ul>
<li><a class="reference internal" href="#reentrant-context-managers">Reentrant context managers</a></li>
<li><a class="reference internal" href="#reusable-context-managers">Reusable context managers</a></li>
</ul>
</li>
</ul>
</li>
</ul>
<h4>Previous topic</h4>
<p class="topless"><a href="dataclasses.html"
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<h4>Next topic</h4>
<p class="topless"><a href="abc.html"
title="next chapter"><code class="xref py py-mod docutils literal notranslate"><span class="pre">abc</span></code> — Abstract Base Classes</a></p>
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