Doc/library/atexit.rst

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

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

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

The :mod:`!atexit` module defines functions to register and unregister
:dfn:`exit handlers`: functions that are automatically executed
"at exit", that is, upon normal program termination (for instance,
if :func:`sys.exit` is called or the main module's execution completes)
or, more generally, upon :term:`interpreter shutdown`.

At exit, all registered exit handlers are called
in the *reverse* order in which they were registered.
If you register ``A``, ``B``, and ``C``, at interpreter shutdown time they
will be run in the order ``C``, ``B``, ``A``.
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 an exit handler, 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.

In programs that use multiple interpreters, each interpreter has its own stack
of exit handlers, which are executed when the interpreter shuts down
(for example, with :meth:`concurrent.interpreters.Interpreter.close` or the
C API :c:func:`Py_EndInterpreter`).
Registration functions in this module only affect the interpreter they are
called from.

**Note:** Exit handlers 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.

.. warning::
   When writing exit handlers, especially in C API extensions, keep in mind
   that other exit handlers may still run arbitrary Python code after you
   clean up.
   Such code should succeed or fail with an exception, rather than crash.

.. versionchanged:: 3.12
   Attempts to start a new thread or :func:`os.fork` a new process
   in an exit handler now leads to :exc:`RuntimeError`.
   Previously, this could cause race conditions between the main Python
   runtime thread freeing thread states while internal :mod:`threading`
   routines or the new process try to use that state, which could lead to
   crashes rather than clean shutdown.

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

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

   Register *func* as an exit handler.
   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.

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

.. function:: unregister(func)

   Remove *func* from the list of exit handlers.
   :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.