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<div class="section" id="module-atexit">
<span id="atexit-exit-handlers"></span><h1>28.9. <a class="reference internal" href="#module-atexit" title="atexit: Register and execute cleanup functions."><code class="xref py py-mod docutils literal notranslate"><span class="pre">atexit</span></code></a> — Exit handlers<a class="headerlink" href="#module-atexit" title="Permalink to this headline">¶</a></h1>
<div class="versionadded">
<p><span class="versionmodified added">New in version 2.0.</span></p>
</div>
<p><strong>Source code:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/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."><code class="xref py py-mod docutils literal notranslate"><span class="pre">atexit</span></code></a> module defines a single function to register cleanup
functions. Functions thus registered are automatically executed upon normal
interpreter termination. <a class="reference internal" href="#module-atexit" title="atexit: Register and execute cleanup functions."><code class="xref py py-mod docutils literal notranslate"><span class="pre">atexit</span></code></a> runs these functions in the <em>reverse</em>
order in which they were registered; if you register <code class="docutils literal notranslate"><span class="pre">A</span></code>, <code class="docutils literal notranslate"><span class="pre">B</span></code>, and <code class="docutils literal notranslate"><span class="pre">C</span></code>,
at interpreter termination time they will be run in the order <code class="docutils literal notranslate"><span class="pre">C</span></code>, <code class="docutils literal notranslate"><span class="pre">B</span></code>,
<code class="docutils literal notranslate"><span class="pre">A</span></code>.</p>
<p><strong>Note:</strong> 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"><code class="xref py py-func docutils literal notranslate"><span class="pre">os._exit()</span></code></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"><code class="xref py py-func docutils literal notranslate"><span class="pre">sys.exitfunc()</span></code></a> variable.</p>
<p>Note: This module is unlikely to work correctly when used with other code that
sets <code class="docutils literal notranslate"><span class="pre">sys.exitfunc</span></code>. In particular, other core Python modules are free to use
<a class="reference internal" href="#module-atexit" title="atexit: Register and execute cleanup functions."><code class="xref py py-mod docutils literal notranslate"><span class="pre">atexit</span></code></a> without the programmer’s knowledge. Authors who use
<code class="docutils literal notranslate"><span class="pre">sys.exitfunc</span></code> should convert their code to use <a class="reference internal" href="#module-atexit" title="atexit: Register and execute cleanup functions."><code class="xref py py-mod docutils literal notranslate"><span class="pre">atexit</span></code></a> instead. The
simplest way to convert code that sets <code class="docutils literal notranslate"><span class="pre">sys.exitfunc</span></code> is to import
<a class="reference internal" href="#module-atexit" title="atexit: Register and execute cleanup functions."><code class="xref py py-mod docutils literal notranslate"><span class="pre">atexit</span></code></a> and register the function that had been bound to <code class="docutils literal notranslate"><span class="pre">sys.exitfunc</span></code>.</p>
<dl class="function">
<dt id="atexit.register">
<code class="descclassname">atexit.</code><code class="descname">register</code><span class="sig-paren">(</span><em>func</em><span class="optional">[</span>, <em>*args</em><span class="optional">[</span>, <em>**kwargs</em><span class="optional">]</span><span class="optional">]</span><span class="sig-paren">)</span><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"><code class="xref py py-func docutils literal notranslate"><span class="pre">register()</span></code></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"><code class="xref py py-func docutils literal notranslate"><span class="pre">sys.exit()</span></code></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"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SystemExit</span></code></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>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 2.6: </span>This function now returns <em>func</em>, which makes it possible to use it as a
decorator.</p>
</div>
</dd></dl>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<dl class="simple">
<dt>Module <a class="reference internal" href="readline.html#module-readline" title="readline: GNU readline support for Python. (Unix)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">readline</span></code></a></dt><dd><p>Useful example of <a class="reference internal" href="#module-atexit" title="atexit: Register and execute cleanup functions."><code class="xref py py-mod docutils literal notranslate"><span class="pre">atexit</span></code></a> to read and write <a class="reference internal" href="readline.html#module-readline" title="readline: GNU readline support for Python. (Unix)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">readline</span></code></a> history files.</p>
</dd>
</dl>
</div>
<div class="section" id="atexit-example">
<span id="id1"></span><h2>28.9.1. <a class="reference internal" href="#module-atexit" title="atexit: Register and execute cleanup functions."><code class="xref py py-mod docutils literal notranslate"><span class="pre">atexit</span></code></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-default notranslate"><div class="highlight"><pre><span></span><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="s2">"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="s2">"counter"</span><span class="p">,</span> <span class="s2">"w"</span><span class="p">)</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="s2">"</span><span class="si">%d</span><span class="s2">"</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"><code class="xref py py-func docutils literal notranslate"><span class="pre">register()</span></code></a> to be
passed along to the registered function when it is called:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><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="nb">print</span> <span class="s1">'Goodbye, </span><span class="si">%s</span><span class="s1">, it was </span><span class="si">%s</span><span class="s1"> 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="s1">'Donny'</span><span class="p">,</span> <span class="s1">'nice'</span><span class="p">)</span>
<span class="c1"># 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="s1">'nice'</span><span class="p">,</span> <span class="n">name</span><span class="o">=</span><span class="s1">'Donny'</span><span class="p">)</span>
</pre></div>
</div>
<p>Usage as a <a class="reference internal" href="../glossary.html#term-decorator"><span class="xref std std-term">decorator</span></a>:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">atexit</span>
<span class="nd">@atexit</span><span class="o">.</span><span class="n">register</span>
<span class="k">def</span> <span class="nf">goodbye</span><span class="p">():</span>
<span class="nb">print</span> <span class="s2">"You are now leaving the Python sector."</span>
</pre></div>
</div>
<p>This only works with functions that can be called without arguments.</p>
</div>
</div>
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<ul>
<li><a class="reference internal" href="#">28.9. <code class="xref py py-mod docutils literal notranslate"><span class="pre">atexit</span></code> — Exit handlers</a><ul>
<li><a class="reference internal" href="#atexit-example">28.9.1. <code class="xref py py-mod docutils literal notranslate"><span class="pre">atexit</span></code> Example</a></li>
</ul>
</li>
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<h4>Previous topic</h4>
<p class="topless"><a href="abc.html"
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<h4>Next topic</h4>
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title="next chapter">28.10. <code class="xref py py-mod docutils literal notranslate"><span class="pre">traceback</span></code> — Print or retrieve a stack traceback</a></p>
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