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>

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

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

The :mod:`atexit` 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 :mod:`atexit` simple.

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.


.. 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`.

   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.

   This function returns *func* which makes it possible to use it as a decorator
   without binding the original name to ``None``.


.. function:: unregister(func)

   Remove a function *func* from the list of functions to be run at interpreter-
   shutdown.  After calling :func:`unregister`, *func* is guaranteed not to be
   called when the interpreter shuts down.


.. 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:
       with open("/tmp/counter") as infile:
           _count = int(infile.read())
   except FileNotFoundError:
       _count = 0

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

   def savecounter():
       with open("/tmp/counter", "w") as outfile:
           outfile.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 obviously only works with functions that don't take arguments.