:mod:!atexit --- Exit handlers

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

---

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.