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<html><body><div class="content"><div class="section" id="the-with-statement">
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<span id="as"></span><span id="with"></span><h2>The <a class="reference internal" href="#with"><tt class="xref std std-keyword docutils literal"><span class="pre">with</span></tt></a> statement</h2>
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<p id="index-16">The <a class="reference internal" href="#with"><tt class="xref std std-keyword docutils literal"><span class="pre">with</span></tt></a> statement is used to wrap the execution of a block with
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methods defined by a context manager (see section <a class="reference internal" href="datamodel.html#context-managers"><em>With Statement Context Managers</em></a>).
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This allows common <a class="reference internal" href="#try"><tt class="xref std std-keyword docutils literal"><span class="pre">try</span></tt></a>...<a class="reference internal" href="#except"><tt class="xref std std-keyword docutils literal"><span class="pre">except</span></tt></a>...<a class="reference internal" href="#finally"><tt class="xref std std-keyword docutils literal"><span class="pre">finally</span></tt></a>
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usage patterns to be encapsulated for convenient reuse.</p>
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<pre>
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<strong id="grammar-token-with_stmt">with_stmt</strong> ::= "with" with_item ("," with_item)* ":" <a class="reference internal" href="#grammar-token-suite"><tt class="xref docutils literal"><span class="pre">suite</span></tt></a>
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<strong id="grammar-token-with_item">with_item</strong> ::= <a class="reference internal" href="expressions.html#grammar-token-expression"><tt class="xref docutils literal"><span class="pre">expression</span></tt></a> ["as" <a class="reference internal" href="simple_stmts.html#grammar-token-target"><tt class="xref docutils literal"><span class="pre">target</span></tt></a>]
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</pre>
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<p>The execution of the <a class="reference internal" href="#with"><tt class="xref std std-keyword docutils literal"><span class="pre">with</span></tt></a> statement with one “item” proceeds as follows:</p>
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<ol class="arabic">
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<li><p class="first">The context expression (the expression given in the <a class="reference internal" href="#grammar-token-with_item"><tt class="xref std std-token docutils literal"><span class="pre">with_item</span></tt></a>) is
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evaluated to obtain a context manager.</p>
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</li>
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<li><p class="first">The context manager’s <a class="reference internal" href="datamodel.html#object.__exit__" title="object.__exit__"><tt class="xref py py-meth docutils literal"><span class="pre">__exit__()</span></tt></a> is loaded for later use.</p>
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</li>
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<li><p class="first">The context manager’s <a class="reference internal" href="datamodel.html#object.__enter__" title="object.__enter__"><tt class="xref py py-meth docutils literal"><span class="pre">__enter__()</span></tt></a> method is invoked.</p>
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</li>
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<li><p class="first">If a target was included in the <a class="reference internal" href="#with"><tt class="xref std std-keyword docutils literal"><span class="pre">with</span></tt></a> statement, the return value
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from <a class="reference internal" href="datamodel.html#object.__enter__" title="object.__enter__"><tt class="xref py py-meth docutils literal"><span class="pre">__enter__()</span></tt></a> is assigned to it.</p>
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<div class="admonition note">
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<p class="first admonition-title">Note</p>
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<p class="last">The <a class="reference internal" href="#with"><tt class="xref std std-keyword docutils literal"><span class="pre">with</span></tt></a> statement guarantees that if the <a class="reference internal" href="datamodel.html#object.__enter__" title="object.__enter__"><tt class="xref py py-meth docutils literal"><span class="pre">__enter__()</span></tt></a>
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method returns without an error, then <a class="reference internal" href="datamodel.html#object.__exit__" title="object.__exit__"><tt class="xref py py-meth docutils literal"><span class="pre">__exit__()</span></tt></a> will always be
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called. Thus, if an error occurs during the assignment to the target list,
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it will be treated the same as an error occurring within the suite would
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be. See step 6 below.</p>
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</div>
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</li>
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<li><p class="first">The suite is executed.</p>
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</li>
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<li><p class="first">The context manager’s <a class="reference internal" href="datamodel.html#object.__exit__" title="object.__exit__"><tt class="xref py py-meth docutils literal"><span class="pre">__exit__()</span></tt></a> method is invoked. If an exception
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caused the suite to be exited, its type, value, and traceback are passed as
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arguments to <a class="reference internal" href="datamodel.html#object.__exit__" title="object.__exit__"><tt class="xref py py-meth docutils literal"><span class="pre">__exit__()</span></tt></a>. Otherwise, three <a class="reference internal" href="../library/constants.html#None" title="None"><tt class="xref py py-const docutils literal"><span class="pre">None</span></tt></a> arguments are
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supplied.</p>
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<p>If the suite was exited due to an exception, and the return value from the
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<a class="reference internal" href="datamodel.html#object.__exit__" title="object.__exit__"><tt class="xref py py-meth docutils literal"><span class="pre">__exit__()</span></tt></a> method was false, the exception is reraised. If the return
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value was true, the exception is suppressed, and execution continues with the
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statement following the <a class="reference internal" href="#with"><tt class="xref std std-keyword docutils literal"><span class="pre">with</span></tt></a> statement.</p>
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<p>If the suite was exited for any reason other than an exception, the return
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value from <a class="reference internal" href="datamodel.html#object.__exit__" title="object.__exit__"><tt class="xref py py-meth docutils literal"><span class="pre">__exit__()</span></tt></a> is ignored, and execution proceeds at the normal
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location for the kind of exit that was taken.</p>
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</li>
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</ol>
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<p>With more than one item, the context managers are processed as if multiple
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<a class="reference internal" href="#with"><tt class="xref std std-keyword docutils literal"><span class="pre">with</span></tt></a> statements were nested:</p>
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<div class="highlight-python3"><div class="highlight"><pre><span class="k">with</span> <span class="n">A</span><span class="p">()</span> <span class="k">as</span> <span class="n">a</span><span class="p">,</span> <span class="n">B</span><span class="p">()</span> <span class="k">as</span> <span class="n">b</span><span class="p">:</span>
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<span class="n">suite</span>
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</pre></div>
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</div>
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<p>is equivalent to</p>
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<div class="highlight-python3"><div class="highlight"><pre><span class="k">with</span> <span class="n">A</span><span class="p">()</span> <span class="k">as</span> <span class="n">a</span><span class="p">:</span>
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<span class="k">with</span> <span class="n">B</span><span class="p">()</span> <span class="k">as</span> <span class="n">b</span><span class="p">:</span>
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<span class="n">suite</span>
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</pre></div>
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</div>
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<div class="versionchanged">
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<p><span class="versionmodified">Changed in version 3.1: </span>Support for multiple context expressions.</p>
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</div>
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<div class="admonition seealso">
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<p class="first admonition-title">See also</p>
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<dl class="last docutils">
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<dt><span class="target" id="index-17"></span><a class="pep reference external" href="http://www.python.org/dev/peps/pep-0343"><strong>PEP 0343</strong></a> - The “with” statement</dt>
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<dd>The specification, background, and examples for the Python <a class="reference internal" href="#with"><tt class="xref std std-keyword docutils literal"><span class="pre">with</span></tt></a>
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statement.</dd>
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</dl>
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</div>
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</div>
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</div></body></html> |