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<div class="section" id="module-contextlib">
<h1>27.5. <tt class="xref docutils literal"><span class="pre">contextlib</span></tt> — Utilities for <a class="reference external" href="../reference/compound_stmts.html#with"><tt class="xref 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>This module provides utilities for common tasks involving the <a class="reference external" href="../reference/compound_stmts.html#with"><tt class="xref docutils literal"><span class="pre">with</span></tt></a>
statement. For more information see also <a class="reference external" href="stdtypes.html#typecontextmanager"><em>Context Manager Types</em></a> and
<a class="reference external" 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">contextlib.</tt><tt class="descname">contextmanager</tt><big>(</big><em>func</em><big>)</big><a class="headerlink" href="#contextlib.contextmanager" title="Permalink to this definition">¶</a></dt>
<dd><p>This function is a <a class="reference external" href="../glossary.html#term-decorator"><em class="xref">decorator</em></a> that can be used to define a factory
function for <a class="reference external" href="../reference/compound_stmts.html#with"><tt class="xref docutils literal"><span class="pre">with</span></tt></a> statement context managers, without needing to
create a class or separate <a title="object.__enter__" class="reference external" href="../reference/datamodel.html#object.__enter__"><tt class="xref docutils literal"><span class="pre">__enter__()</span></tt></a> and <a title="object.__exit__" class="reference external" href="../reference/datamodel.html#object.__exit__"><tt class="xref 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 external" href="../glossary.html#term-generator"><em class="xref">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 external" href="../reference/compound_stmts.html#with"><tt class="xref docutils literal"><span class="pre">with</span></tt></a> statement’s <a class="reference external" href="../reference/compound_stmts.html#as"><tt class="xref 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 external" href="../reference/compound_stmts.html#with"><tt class="xref 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 external" href="../reference/compound_stmts.html#try"><tt class="xref docutils literal"><span class="pre">try</span></tt></a>...<a class="reference external" href="../reference/compound_stmts.html#except"><tt class="xref docutils literal"><span class="pre">except</span></tt></a>...<a class="reference external" href="../reference/compound_stmts.html#finally"><tt class="xref 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 external" href="../reference/compound_stmts.html#with"><tt class="xref 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 external" href="../reference/compound_stmts.html#with"><tt class="xref docutils literal"><span class="pre">with</span></tt></a> statement.</p>
</dd></dl>
<dl class="function">
<dt id="contextlib.nested">
<tt class="descclassname">contextlib.</tt><tt class="descname">nested</tt><big>(</big><em>mgr1</em><span class="optional">[</span>, <em>mgr2</em><span class="optional">[</span>, <em>...</em><span class="optional">]</span><span class="optional">]</span><big>)</big><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 external" href="../reference/compound_stmts.html#with"><tt class="xref docutils literal"><span class="pre">with</span></tt></a> statement.</p>
<p>The one advantage of this function over the multiple manager form of the
<a class="reference external" href="../reference/compound_stmts.html#with"><tt class="xref docutils literal"><span class="pre">with</span></tt></a> statement is that argument unpacking allows it to be
used with a variable number of context managers as follows:</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">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 title="object.__exit__" class="reference external" href="../reference/datamodel.html#object.__exit__"><tt class="xref docutils literal"><span class="pre">__exit__()</span></tt></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 title="object.__exit__" class="reference external" href="../reference/datamodel.html#object.__exit__"><tt class="xref docutils literal"><span class="pre">__exit__()</span></tt></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 title="object.__exit__" class="reference external" href="../reference/datamodel.html#object.__exit__"><tt class="xref docutils literal"><span class="pre">__exit__()</span></tt></a> methods of any remaining outer context managers. In general,
<a title="object.__exit__" class="reference external" href="../reference/datamodel.html#object.__exit__"><tt class="xref docutils literal"><span class="pre">__exit__()</span></tt></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 title="object.__new__" class="reference external" href="../reference/datamodel.html#object.__new__"><tt class="xref docutils literal"><span class="pre">__new__()</span></tt></a> and <a title="object.__init__" class="reference external" href="../reference/datamodel.html#object.__init__"><tt class="xref docutils literal"><span class="pre">__init__()</span></tt></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 title="contextlib.nested" class="reference internal" href="#contextlib.nested"><tt class="xref docutils literal"><span class="pre">nested()</span></tt></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 title="object.__enter__" class="reference external" href="../reference/datamodel.html#object.__enter__"><tt class="xref docutils literal"><span class="pre">__enter__()</span></tt></a> method of one of the inner context managers
raises an exception that is caught and suppressed by the <a title="object.__exit__" class="reference external" href="../reference/datamodel.html#object.__exit__"><tt class="xref docutils literal"><span class="pre">__exit__()</span></tt></a> method
of one of the outer context managers, this construct will raise
<a title="exceptions.RuntimeError" class="reference external" href="exceptions.html#exceptions.RuntimeError"><tt class="xref docutils literal"><span class="pre">RuntimeError</span></tt></a> rather than skipping the body of the <a class="reference external" href="../reference/compound_stmts.html#with"><tt class="xref docutils literal"><span class="pre">with</span></tt></a>
statement.</p>
<p>Developers that need to support nesting of a variable number of context managers
can either use the <a title="Issue warning messages and control their disposition." class="reference external" href="warnings.html#module-warnings"><tt class="xref docutils literal"><span class="pre">warnings</span></tt></a> module to suppress the DeprecationWarning
raised by this function or else use this function as a model for an application
specific implementation.</p>
<p>
<span class="versionmodified">Deprecated since version 3.1: </span>The with-statement now supports this functionality directly (without the
confusing error prone quirks).</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 external" href="../reference/compound_stmts.html#with"><tt class="xref docutils literal"><span class="pre">with</span></tt></a> block is exited.</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-210"></span><a class="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 external" href="../reference/compound_stmts.html#with"><tt class="xref docutils literal"><span class="pre">with</span></tt></a>
statement.</dd>
</dl>
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