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<div class="section" id="module-contextlib">
<span id="contextlib-utilities-for-with-statement-contexts"></span><h1>27.6. <a class="reference internal" href="#module-contextlib" title="contextlib: Utilities for with-statement contexts."><tt class="xref py py-mod docutils literal"><span class="pre">contextlib</span></tt></a> — Utilities for <a class="reference internal" href="../reference/compound_stmts.html#with"><tt class="xref std std-keyword docutils literal"><span class="pre">with</span></tt></a>-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="http://hg.python.org/cpython/file/3.2/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"><tt class="xref std std-keyword docutils literal"><span class="pre">with</span></tt></a>
statement. For more information see also <a class="reference internal" href="stdtypes.html#typecontextmanager"><em>Context Manager Types</em></a> and
<a class="reference internal" href="../reference/datamodel.html#context-managers"><em>With Statement Context Managers</em></a>.</p>
<p>Functions provided:</p>
<dl class="function">
<dt id="contextlib.contextmanager">
<tt class="descclassname">@</tt><tt class="descclassname">contextlib.</tt><tt class="descname">contextmanager</tt><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"><em class="xref std std-term">decorator</em></a> that can be used to define a factory
function for <a class="reference internal" href="../reference/compound_stmts.html#with"><tt class="xref std std-keyword docutils literal"><span class="pre">with</span></tt></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__"><tt class="xref py py-meth docutils literal"><span class="pre">__enter__()</span></tt></a> and <a class="reference internal" href="../reference/datamodel.html#object.__exit__" title="object.__exit__"><tt class="xref py py-meth docutils literal"><span class="pre">__exit__()</span></tt></a> methods.</p>
<p>A simple example (this is not recommended as a real way of generating HTML!):</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">contextlib</span> <span class="k">import</span> <span class="n">contextmanager</span>
<span class="nd">@contextmanager</span>
<span class="k">def</span> <span class="nf">tag</span><span class="p">(</span><span class="n">name</span><span class="p">):</span>
<span class="nb">print</span><span class="p">(</span><span class="s">"<%s>"</span> <span class="o">%</span> <span class="n">name</span><span class="p">)</span>
<span class="k">yield</span>
<span class="nb">print</span><span class="p">(</span><span class="s">"</%s>"</span> <span class="o">%</span> <span class="n">name</span><span class="p">)</span>
<span class="o">>>></span> <span class="k">with</span> <span class="n">tag</span><span class="p">(</span><span class="s">"h1"</span><span class="p">):</span>
<span class="o">...</span> <span class="nb">print</span><span class="p">(</span><span class="s">"foo"</span><span class="p">)</span>
<span class="o">...</span>
<span class="o"><</span><span class="n">h1</span><span class="o">></span>
<span class="n">foo</span>
<span class="o"></</span><span class="n">h1</span><span class="o">></span>
</pre></div>
</div>
<p>The function being decorated must return a <a class="reference internal" href="../glossary.html#term-generator"><em class="xref std std-term">generator</em></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"><tt class="xref std std-keyword docutils literal"><span class="pre">with</span></tt></a> statement’s <a class="reference internal" href="../reference/compound_stmts.html#as"><tt class="xref std std-keyword docutils literal"><span class="pre">as</span></tt></a> 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"><tt class="xref std std-keyword docutils literal"><span class="pre">with</span></tt></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"><tt class="xref std std-keyword docutils literal"><span class="pre">try</span></tt></a>...<a class="reference internal" href="../reference/compound_stmts.html#except"><tt class="xref std std-keyword docutils literal"><span class="pre">except</span></tt></a>...<a class="reference internal" href="../reference/compound_stmts.html#finally"><tt class="xref std std-keyword docutils literal"><span class="pre">finally</span></tt></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 <a class="reference internal" href="../reference/compound_stmts.html#with"><tt class="xref std std-keyword docutils literal"><span class="pre">with</span></tt></a> statement that
the exception has been handled, and execution will resume with the statement
immediately following the <a class="reference internal" href="../reference/compound_stmts.html#with"><tt class="xref std std-keyword docutils literal"><span class="pre">with</span></tt></a> statement.</p>
<p><a class="reference internal" href="#contextlib.contextmanager" title="contextlib.contextmanager"><tt class="xref py py-func docutils literal"><span class="pre">contextmanager()</span></tt></a> uses <a class="reference internal" href="#contextlib.ContextDecorator" title="contextlib.ContextDecorator"><tt class="xref py py-class docutils literal"><span class="pre">ContextDecorator</span></tt></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"><tt class="xref std std-keyword docutils literal"><span class="pre">with</span></tt></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"><tt class="xref py py-func docutils literal"><span class="pre">contextmanager()</span></tt></a> to meet the requirement that context
managers support multiple invocations in order to be used as decorators).</p>
<p class="versionchanged">
<span class="versionmodified">Changed in version 3.2: </span>Use of <a class="reference internal" href="#contextlib.ContextDecorator" title="contextlib.ContextDecorator"><tt class="xref py py-class docutils literal"><span class="pre">ContextDecorator</span></tt></a>.</p>
</dd></dl>
<dl class="function">
<dt id="contextlib.closing">
<tt class="descclassname">contextlib.</tt><tt class="descname">closing</tt><big>(</big><em>thing</em><big>)</big><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"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">contextlib</span> <span class="k">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"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">contextlib</span> <span class="k">import</span> <span class="n">closing</span>
<span class="kn">from</span> <span class="nn">urllib.request</span> <span class="k">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="s">'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 <tt class="docutils literal"><span class="pre">page</span></tt>. Even if an error occurs,
<tt class="docutils literal"><span class="pre">page.close()</span></tt> will be called when the <a class="reference internal" href="../reference/compound_stmts.html#with"><tt class="xref std std-keyword docutils literal"><span class="pre">with</span></tt></a> block is exited.</p>
</dd></dl>
<dl class="class">
<dt id="contextlib.ContextDecorator">
<em class="property">class </em><tt class="descclassname">contextlib.</tt><tt class="descname">ContextDecorator</tt><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 <tt class="docutils literal"><span class="pre">ContextDecorator</span></tt> have to implement
<tt class="docutils literal"><span class="pre">__enter__</span></tt> and <tt class="docutils literal"><span class="pre">__exit__</span></tt> as normal. <tt class="docutils literal"><span class="pre">__exit__</span></tt> retains its optional
exception handling even when used as a decorator.</p>
<p><tt class="docutils literal"><span class="pre">ContextDecorator</span></tt> is used by <a class="reference internal" href="#contextlib.contextmanager" title="contextlib.contextmanager"><tt class="xref py py-func docutils literal"><span class="pre">contextmanager()</span></tt></a>, so you get this
functionality automatically.</p>
<p>Example of <tt class="docutils literal"><span class="pre">ContextDecorator</span></tt>:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">contextlib</span> <span class="k">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="nf">__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="s">'Starting'</span><span class="p">)</span>
<span class="k">return</span> <span class="bp">self</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="o">*</span><span class="n">exc</span><span class="p">):</span>
<span class="nb">print</span><span class="p">(</span><span class="s">'Finishing'</span><span class="p">)</span>
<span class="k">return</span> <span class="k">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="s">'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="s">'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"><div class="highlight"><pre><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="c"># Do stuff</span>
</pre></div>
</div>
<p><tt class="docutils literal"><span class="pre">ContextDecorator</span></tt> lets you instead write:</p>
<div class="highlight-python3"><div class="highlight"><pre><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="c"># Do stuff</span>
</pre></div>
</div>
<p>It makes it clear that the <tt class="docutils literal"><span class="pre">cm</span></tt> 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 <tt class="docutils literal"><span class="pre">ContextDecorator</span></tt> as a mixin class:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">contextlib</span> <span class="k">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="nf">__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="nf">__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="k">False</span>
</pre></div>
</div>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">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"><tt class="xref std std-keyword docutils literal"><span class="pre">with</span></tt></a>
statements. If this is not the case, then the original construct with the
explicit <a class="reference internal" href="../reference/compound_stmts.html#with"><tt class="xref std std-keyword docutils literal"><span class="pre">with</span></tt></a> statement inside the function should be used.</p>
</div>
<p class="versionadded">
<span class="versionmodified">New in version 3.2.</span></p>
</dd></dl>
<div class="admonition-see-also admonition seealso">
<p class="first admonition-title">See also</p>
<dl class="last docutils">
<dt><span class="target" id="index-0"></span><a class="pep reference external" href="http://www.python.org/dev/peps/pep-0343"><strong>PEP 0343</strong></a> - The “with” statement</dt>
<dd>The specification, background, and examples for the Python <a class="reference internal" href="../reference/compound_stmts.html#with"><tt class="xref std std-keyword docutils literal"><span class="pre">with</span></tt></a>
statement.</dd>
</dl>
</div>
</div>
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