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<div class="section" id="module-atexit">
<span id="atexit-exit-handlers"></span><h1>27.8. <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><strong>Source code:</strong> <a class="reference external" href="http://hg.python.org/cpython/file/3.2/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 functions to register and unregister 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>
<dl class="function">
<dt id="atexit.register">
<tt class="descclassname">atexit.</tt><tt class="descname">register</tt><big>(</big><em>func</em>, <em>*args</em>, <em>**kargs</em><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>.</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#SystemExit" title="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>This function returns <em>func</em> which makes it possible to use it as a decorator
without binding the original name to <tt class="xref docutils literal"><span class="pre">None</span></tt>.</p>
</dd></dl>
<dl class="function">
<dt id="atexit.unregister">
<tt class="descclassname">atexit.</tt><tt class="descname">unregister</tt><big>(</big><em>func</em><big>)</big><a class="headerlink" href="#atexit.unregister" title="Permalink to this definition">¶</a></dt>
<dd><p>Remove a function <em>func</em> from the list of functions to be run at interpreter-
shutdown. After calling <a class="reference internal" href="#atexit.unregister" title="atexit.unregister"><tt class="xref py py-func docutils literal"><span class="pre">unregister()</span></tt></a>, <em>func</em> is guaranteed not to be
called when the interpreter shuts down.</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>27.8.1. <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-python3"><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">"/tmp/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">"/tmp/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">"%d"</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-python3"><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="nb">print</span><span class="p">(</span><span class="s">'Goodbye, %s, it was %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-python3"><div class="highlight"><pre><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="p">(</span><span class="s">"You are now leaving the Python sector."</span><span class="p">)</span>
</pre></div>
</div>
<p>This obviously only works with functions that don’t take arguments.</p>
</div>
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<li><a class="reference internal" href="#">27.8. <tt class="docutils literal"><span class="pre">atexit</span></tt> — Exit handlers</a><ul>
<li><a class="reference internal" href="#atexit-example">27.8.1. <tt class="docutils literal"><span class="pre">atexit</span></tt> Example</a></li>
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