Doc/library/atexit.rst

:mod:`atexit` --- Exit handlers
===============================

.. module:: atexit
   :synopsis: Register and execute cleanup functions.
.. moduleauthor:: Skip Montanaro <skip@pobox.com>
.. sectionauthor:: Skip Montanaro <skip@pobox.com>


.. versionadded:: 2.0

**Source code:** :source:`Lib/atexit.py`

--------------

The :mod:`atexit` module defines a single function to register cleanup
functions.  Functions thus registered are automatically executed upon normal
interpreter termination.  :mod:`atexit` runs these functions in the *reverse*
order in which they were registered; if you register ``A``, ``B``, and ``C``,
at interpreter termination time they will be run in the order ``C``, ``B``,
``A``.

**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 :func:`os._exit` is called.

.. index:: single: exitfunc (in sys)

This is an alternate interface to the functionality provided by the
:func:`sys.exitfunc` variable.

Note: This module is unlikely to work correctly when used with other code that
sets ``sys.exitfunc``.  In particular, other core Python modules are free to use
:mod:`atexit` without the programmer's knowledge.  Authors who use
``sys.exitfunc`` should convert their code to use :mod:`atexit` instead.  The
simplest way to convert code that sets ``sys.exitfunc`` is to import
:mod:`atexit` and register the function that had been bound to ``sys.exitfunc``.


.. function:: register(func[, *args[, **kargs]])

   Register *func* as a function to be executed at termination.  Any optional
   arguments that are to be passed to *func* must be passed as arguments to
   :func:`register`.  It is possible to register the same function and arguments
   more than once.

   At normal program termination (for instance, if :func:`sys.exit` 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.

   If an exception is raised during execution of the exit handlers, a traceback is
   printed (unless :exc:`SystemExit` 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.

   .. versionchanged:: 2.6
      This function now returns *func*, which makes it possible to use it as a
      decorator.


.. seealso::

   Module :mod:`readline`
      Useful example of :mod:`atexit` to read and write :mod:`readline` history files.


.. _atexit-example:

:mod:`atexit` Example
---------------------

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. ::

   try:
       _count = int(open("counter").read())
   except IOError:
       _count = 0

   def incrcounter(n):
       global _count
       _count = _count + n

   def savecounter():
       open("counter", "w").write("%d" % _count)

   import atexit
   atexit.register(savecounter)

Positional and keyword arguments may also be passed to :func:`register` to be
passed along to the registered function when it is called::

   def goodbye(name, adjective):
       print 'Goodbye, %s, it was %s to meet you.' % (name, adjective)

   import atexit
   atexit.register(goodbye, 'Donny', 'nice')

   # or:
   atexit.register(goodbye, adjective='nice', name='Donny')

Usage as a :term:`decorator`::

   import atexit

   @atexit.register
   def goodbye():
       print "You are now leaving the Python sector."

This only works with functions that can be called without arguments.