Edit File: contextlib.html
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Python Runtime Services</a> »</li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body"> <div class="section" id="module-contextlib"> <span id="contextlib-utilities-for-with-statement-contexts"></span><h1>27.7. <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 class="versionadded"> <span class="versionmodified">New in version 2.5.</span></p> <p><strong>Source code:</strong> <a class="reference external" href="http://hg.python.org/cpython/file/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"><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">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 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-python"><pre>from contextlib import contextmanager @contextmanager def tag(name): print "<%s>" % name yield print "</%s>" % name >>> with tag("h1"): ... print "foo" ... <h1> foo </h1></pre> </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> </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 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>The one advantage of this function over the multiple manager form of 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 that argument unpacking allows it to be used with a variable number of context managers as follows:</p> <div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">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__"><tt class="xref py py-meth 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 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> 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__"><tt class="xref py py-meth docutils literal"><span class="pre">__exit__()</span></tt></a> methods of any remaining outer context managers. In general, <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 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__"><tt class="xref py py-meth docutils literal"><span class="pre">__new__()</span></tt></a> and <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><tt class="xref py py-meth 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 class="reference internal" href="#contextlib.nested" title="contextlib.nested"><tt class="xref py py-func 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 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> 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__"><tt class="xref py py-meth docutils literal"><span class="pre">__exit__()</span></tt></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"><tt class="xref py py-exc docutils literal"><span class="pre">RuntimeError</span></tt></a> rather than skipping the body of 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>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."><tt class="xref py py-mod 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 class="deprecated"> <span class="versionmodified">Deprecated since version 2.7: </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-python"><div class="highlight"><pre><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-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">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="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="k">print</span> <span class="n">line</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> <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> </div> </div> </div> <div class="sphinxsidebar"> <div class="sphinxsidebarwrapper"> <h4>Previous topic</h4> <p class="topless"><a href="warnings.html" title="previous chapter">27.6. <tt class="docutils literal"><span class="pre">warnings</span></tt> — Warning control</a></p> <h4>Next topic</h4> <p class="topless"><a href="abc.html" title="next chapter">27.8. <tt class="docutils literal"><span class="pre">abc</span></tt> — Abstract Base Classes</a></p> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../bugs.html">Report a Bug</a></li> <li><a href="../_sources/library/contextlib.txt" rel="nofollow">Show Source</a></li> </ul> <div id="searchbox" style="display: none"> <h3>Quick search</h3> <form class="search" action="../search.html" method="get"> <input type="text" name="q" /> <input type="submit" value="Go" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> <p class="searchtip" style="font-size: 90%"> Enter search terms or a module, class or function name. </p> </div> <script type="text/javascript">$('#searchbox').show(0);</script> </div> </div> <div class="clearer"></div> </div> <div class="related"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" >index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="abc.html" title="27.8. abc — Abstract Base Classes" >next</a> |</li> <li class="right" > <a href="warnings.html" title="27.6. warnings — Warning control" >previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="http://www.python.org/">Python</a> »</li> <li> <a href="../index.html">Python 2.7.5 documentation</a> » </li> <li><a href="index.html" >The Python Standard Library</a> »</li> <li><a href="python.html" >27. 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