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<div class="section" id="module-atexit">
<span id="atexit-exit-handlers"></span><h1>27.9. <a class="reference internal" href="#module-atexit" title="atexit: Register and execute cleanup functions."><tt class="xref py py-mod docutils literal"><span class="pre">atexit</span></tt></a> — Exit handlers<a class="headerlink" href="#module-atexit" title="Permalink to this headline">¶</a></h1>
<p class="versionadded">
<span class="versionmodified">New in version 2.0.</span></p>
<p><strong>Source code:</strong> <a class="reference external" href="http://hg.python.org/cpython/file/2.7/Lib/atexit.py">Lib/atexit.py</a></p>
<hr class="docutils" />
<p>The <a class="reference internal" href="#module-atexit" title="atexit: Register and execute cleanup functions."><tt class="xref py py-mod docutils literal"><span class="pre">atexit</span></tt></a> module defines a single function to register cleanup
functions. Functions thus registered are automatically executed upon normal
interpreter termination. The order in which the functions are called is not
defined; if you have cleanup operations that depend on each other, you should
wrap them in a function and register that one. This keeps <a class="reference internal" href="#module-atexit" title="atexit: Register and execute cleanup functions."><tt class="xref py py-mod docutils literal"><span class="pre">atexit</span></tt></a> simple.</p>
<p>Note: the functions registered via this module are not called when the program
is killed by a signal not handled by Python, when a Python fatal internal error
is detected, or when <a class="reference internal" href="os.html#os._exit" title="os._exit"><tt class="xref py py-func docutils literal"><span class="pre">os._exit()</span></tt></a> is called.</p>
<p id="index-0">This is an alternate interface to the functionality provided by the
<a class="reference internal" href="sys.html#sys.exitfunc" title="sys.exitfunc"><tt class="xref py py-func docutils literal"><span class="pre">sys.exitfunc()</span></tt></a> variable.</p>
<p>Note: This module is unlikely to work correctly when used with other code that
sets <tt class="docutils literal"><span class="pre">sys.exitfunc</span></tt>. In particular, other core Python modules are free to use
<a class="reference internal" href="#module-atexit" title="atexit: Register and execute cleanup functions."><tt class="xref py py-mod docutils literal"><span class="pre">atexit</span></tt></a> without the programmer’s knowledge. Authors who use
<tt class="docutils literal"><span class="pre">sys.exitfunc</span></tt> should convert their code to use <a class="reference internal" href="#module-atexit" title="atexit: Register and execute cleanup functions."><tt class="xref py py-mod docutils literal"><span class="pre">atexit</span></tt></a> instead. The
simplest way to convert code that sets <tt class="docutils literal"><span class="pre">sys.exitfunc</span></tt> is to import
<a class="reference internal" href="#module-atexit" title="atexit: Register and execute cleanup functions."><tt class="xref py py-mod docutils literal"><span class="pre">atexit</span></tt></a> and register the function that had been bound to <tt class="docutils literal"><span class="pre">sys.exitfunc</span></tt>.</p>
<dl class="function">
<dt id="atexit.register">
<tt class="descclassname">atexit.</tt><tt class="descname">register</tt><big>(</big><em>func</em><span class="optional">[</span>, <em>*args</em><span class="optional">[</span>, <em>**kargs</em><span class="optional">]</span><span class="optional">]</span><big>)</big><a class="headerlink" href="#atexit.register" title="Permalink to this definition">¶</a></dt>
<dd><p>Register <em>func</em> as a function to be executed at termination. Any optional
arguments that are to be passed to <em>func</em> must be passed as arguments to
<a class="reference internal" href="#atexit.register" title="atexit.register"><tt class="xref py py-func docutils literal"><span class="pre">register()</span></tt></a>. It is possible to register the same function and arguments
more than once.</p>
<p>At normal program termination (for instance, if <a class="reference internal" href="sys.html#sys.exit" title="sys.exit"><tt class="xref py py-func docutils literal"><span class="pre">sys.exit()</span></tt></a> is called or
the main module’s execution completes), all functions registered are called in
last in, first out order. The assumption is that lower level modules will
normally be imported before higher level modules and thus must be cleaned up
later.</p>
<p>If an exception is raised during execution of the exit handlers, a traceback is
printed (unless <a class="reference internal" href="exceptions.html#exceptions.SystemExit" title="exceptions.SystemExit"><tt class="xref py py-exc docutils literal"><span class="pre">SystemExit</span></tt></a> is raised) and the exception information is
saved. After all exit handlers have had a chance to run the last exception to
be raised is re-raised.</p>
<p class="versionchanged">
<span class="versionmodified">Changed in version 2.6: </span>This function now returns <em>func</em>, which makes it possible to use it as a
decorator.</p>
</dd></dl>
<div class="admonition-see-also admonition seealso">
<p class="first admonition-title">See also</p>
<dl class="last docutils">
<dt>Module <a class="reference internal" href="readline.html#module-readline" title="readline: GNU readline support for Python. (Unix)"><tt class="xref py py-mod docutils literal"><span class="pre">readline</span></tt></a></dt>
<dd>Useful example of <a class="reference internal" href="#module-atexit" title="atexit: Register and execute cleanup functions."><tt class="xref py py-mod docutils literal"><span class="pre">atexit</span></tt></a> to read and write <a class="reference internal" href="readline.html#module-readline" title="readline: GNU readline support for Python. (Unix)"><tt class="xref py py-mod docutils literal"><span class="pre">readline</span></tt></a> history files.</dd>
</dl>
</div>
<div class="section" id="atexit-example">
<span id="id1"></span><h2>27.9.1. <a class="reference internal" href="#module-atexit" title="atexit: Register and execute cleanup functions."><tt class="xref py py-mod docutils literal"><span class="pre">atexit</span></tt></a> Example<a class="headerlink" href="#atexit-example" title="Permalink to this headline">¶</a></h2>
<p>The following simple example demonstrates how a module can initialize a counter
from a file when it is imported and save the counter’s updated value
automatically when the program terminates without relying on the application
making an explicit call into this module at termination.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">try</span><span class="p">:</span>
<span class="n">_count</span> <span class="o">=</span> <span class="nb">int</span><span class="p">(</span><span class="nb">open</span><span class="p">(</span><span class="s">"counter"</span><span class="p">)</span><span class="o">.</span><span class="n">read</span><span class="p">())</span>
<span class="k">except</span> <span class="ne">IOError</span><span class="p">:</span>
<span class="n">_count</span> <span class="o">=</span> <span class="mi">0</span>
<span class="k">def</span> <span class="nf">incrcounter</span><span class="p">(</span><span class="n">n</span><span class="p">):</span>
<span class="k">global</span> <span class="n">_count</span>
<span class="n">_count</span> <span class="o">=</span> <span class="n">_count</span> <span class="o">+</span> <span class="n">n</span>
<span class="k">def</span> <span class="nf">savecounter</span><span class="p">():</span>
<span class="nb">open</span><span class="p">(</span><span class="s">"counter"</span><span class="p">,</span> <span class="s">"w"</span><span class="p">)</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="s">"</span><span class="si">%d</span><span class="s">"</span> <span class="o">%</span> <span class="n">_count</span><span class="p">)</span>
<span class="kn">import</span> <span class="nn">atexit</span>
<span class="n">atexit</span><span class="o">.</span><span class="n">register</span><span class="p">(</span><span class="n">savecounter</span><span class="p">)</span>
</pre></div>
</div>
<p>Positional and keyword arguments may also be passed to <a class="reference internal" href="#atexit.register" title="atexit.register"><tt class="xref py py-func docutils literal"><span class="pre">register()</span></tt></a> to be
passed along to the registered function when it is called:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">def</span> <span class="nf">goodbye</span><span class="p">(</span><span class="n">name</span><span class="p">,</span> <span class="n">adjective</span><span class="p">):</span>
<span class="k">print</span> <span class="s">'Goodbye, </span><span class="si">%s</span><span class="s">, it was </span><span class="si">%s</span><span class="s"> to meet you.'</span> <span class="o">%</span> <span class="p">(</span><span class="n">name</span><span class="p">,</span> <span class="n">adjective</span><span class="p">)</span>
<span class="kn">import</span> <span class="nn">atexit</span>
<span class="n">atexit</span><span class="o">.</span><span class="n">register</span><span class="p">(</span><span class="n">goodbye</span><span class="p">,</span> <span class="s">'Donny'</span><span class="p">,</span> <span class="s">'nice'</span><span class="p">)</span>
<span class="c"># or:</span>
<span class="n">atexit</span><span class="o">.</span><span class="n">register</span><span class="p">(</span><span class="n">goodbye</span><span class="p">,</span> <span class="n">adjective</span><span class="o">=</span><span class="s">'nice'</span><span class="p">,</span> <span class="n">name</span><span class="o">=</span><span class="s">'Donny'</span><span class="p">)</span>
</pre></div>
</div>
<p>Usage as a <a class="reference internal" href="../glossary.html#term-decorator"><em class="xref std std-term">decorator</em></a>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">atexit</span>
<span class="nd">@atexit.register</span>
<span class="k">def</span> <span class="nf">goodbye</span><span class="p">():</span>
<span class="k">print</span> <span class="s">"You are now leaving the Python sector."</span>
</pre></div>
</div>
<p>This only works with functions that can be called without arguments.</p>
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<ul>
<li><a class="reference internal" href="#">27.9. <tt class="docutils literal"><span class="pre">atexit</span></tt> — Exit handlers</a><ul>
<li><a class="reference internal" href="#atexit-example">27.9.1. <tt class="docutils literal"><span class="pre">atexit</span></tt> Example</a></li>
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