Doc/library/atexit.rst

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

.. module:: atexit
   :synopsis: Register and execute cleanup functions.

.. moduleauthor:: Skip Montanaro <skip.montanaro@gmail.com>
.. sectionauthor:: Skip Montanaro <skip.montanaro@gmail.com>

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

The :mod:`atexit` module defines functions to register and unregister 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.

**Note:** The effect of registering or unregistering functions from within
a cleanup function is undefined.

.. versionchanged:: 3.7
    When used with C-API subinterpreters, registered functions
    are local to the interpreter they were registered in.

.. function:: register(func, *args, **kwargs)

   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.

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

   .. warning::
       Starting new threads or calling :func:`os.fork` from a registered
       function can lead to race condition between the main Python
       runtime thread freeing thread states while internal :mod:`threading`
       routines or the new process try to use that state. This can lead to
       crashes rather than clean shutdown.

   .. versionchanged:: 3.12
       Attempts to start a new thread or :func:`os.fork` a new process
       in a registered function now leads to :exc:`RuntimeError`.

.. function:: unregister(func)

   Remove *func* from the list of functions to be run at interpreter shutdown.
   :func:`unregister` silently does nothing if *func* was not previously
   registered.  If *func* has been registered more than once, every occurrence
   of that function in the :mod:`atexit` call stack will be removed.  Equality
   comparisons (``==``) are used internally during unregistration, so function
   references do not need to have matching identities.


.. 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('counterfile') as infile:
           _count = int(infile.read())
   except FileNotFoundError:
       _count = 0

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

   def savecounter():
       with open('counterfile', '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 only works with functions that can be called without arguments.