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<div class="section" id="module-contextlib">
<span id="contextlib-utilities-for-with-statement-contexts"></span><h1>28.7. <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> — Utilities 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 contexts<a class="headerlink" href="#module-contextlib" title="Permalink to this headline">¶</a></h1>
<div class="versionadded">
<p><span class="versionmodified">New in version 2.5.</span></p>
</div>
<p><strong>Source code:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/2.7/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>
<p>Functions provided:</p>
<dl class="function">
<dt id="contextlib.contextmanager">
<code class="descclassname">contextlib.</code><code class="descname">contextmanager</code><span class="sig-paren">(</span><em>func</em><span class="sig-paren">)</span><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-default notranslate"><div class="highlight"><pre><span></span><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">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 <a class="reference internal" href="../reference/compound_stmts.html#as"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">as</span></code></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"><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 <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 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"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement.</p>
</dd></dl>
<dl class="function">
<dt id="contextlib.nested">
<code class="descclassname">contextlib.</code><code class="descname">nested</code><span class="sig-paren">(</span><em>mgr1</em><span class="optional">[</span>, <em>mgr2</em><span class="optional">[</span>, <em>...</em><span class="optional">]</span><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#contextlib.nested" title="Permalink to this definition">¶</a></dt>
<dd><p>Combine multiple context managers into a single nested context manager.</p>
<p>This function has been deprecated in favour of the multiple manager form
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.</p>
<p>The one advantage of this function over the multiple manager form 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 is that argument unpacking allows it to be
used with a variable number of context managers as follows:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="k">import</span> <span class="n">nested</span>
<span class="k">with</span> <span class="n">nested</span><span class="p">(</span><span class="o">*</span><span class="n">managers</span><span class="p">):</span>
<span class="n">do_something</span><span class="p">()</span>
</pre></div>
</div>
<p>Note that if the <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 of one of the nested context managers
indicates an exception should be suppressed, no exception information will be
passed to any remaining outer context managers. Similarly, if the
<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 of one of the nested managers raises an exception, any
previous exception state will be lost; the new exception will be passed to the
<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 of any remaining outer context managers. In general,
<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 should avoid raising exceptions, and in particular they
should not re-raise a passed-in exception.</p>
<p>This function has two major quirks that have led to it being deprecated. Firstly,
as the context managers are all constructed before the function is invoked, the
<a class="reference internal" href="../reference/datamodel.html#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__new__()</span></code></a> and <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> methods of the inner context managers are
not actually covered by the scope of the outer context managers. That means, for
example, that using <a class="reference internal" href="#contextlib.nested" title="contextlib.nested"><code class="xref py py-func docutils literal notranslate"><span class="pre">nested()</span></code></a> to open two files is a programming error as the
first file will not be closed promptly if an exception is thrown when opening
the second file.</p>
<p>Secondly, if 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> method of one of the inner context managers
raises an exception that is caught and suppressed by the <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
of one of the outer context managers, this construct will raise
<a class="reference internal" href="exceptions.html#exceptions.RuntimeError" title="exceptions.RuntimeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">RuntimeError</span></code></a> rather than skipping the body 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.</p>
<p>Developers that need to support nesting of a variable number of context managers
can either use the <a class="reference internal" href="warnings.html#module-warnings" title="warnings: Issue warning messages and control their disposition."><code class="xref py py-mod docutils literal notranslate"><span class="pre">warnings</span></code></a> module to suppress the DeprecationWarning
raised by this function or else use this function as a model for an application
specific implementation.</p>
<div class="deprecated">
<p><span class="versionmodified">Deprecated since version 2.7: </span>The with-statement now supports this functionality directly (without the
confusing error prone quirks).</p>
</div>
</dd></dl>
<dl class="function">
<dt id="contextlib.closing">
<code class="descclassname">contextlib.</code><code class="descname">closing</code><span class="sig-paren">(</span><em>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-default notranslate"><div class="highlight"><pre><span></span><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-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="k">import</span> <span class="n">closing</span>
<span class="kn">import</span> <span class="nn">urllib</span>
<span class="k">with</span> <span class="n">closing</span><span class="p">(</span><span class="n">urllib</span><span class="o">.</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="n">line</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>
<div class="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="https://www.python.org/dev/peps/pep-0343"><strong>PEP 343</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"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a>
statement.</dd>
</dl>
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