atexit --- 終了ハンドラー

The atexit module defines functions to register and unregister exit handlers: functions that are automatically executed "at exit", that is, upon normal program termination (for instance, if sys.exit() is called or the main module's execution completes) or, more generally, upon 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 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 concurrent.interpreters.Interpreter.close() or the C API 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 os._exit() is called.

注意: クリーンアップ関数内からの関数の登録・登録解除効果は定義されていません。

警告

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.

バージョン 3.12 で変更: Attempts to start a new thread or os.fork() a new process in an exit handler now leads to RuntimeError. Previously, this could cause race conditions between the main Python runtime thread freeing thread states while internal threading routines or the new process try to use that state, which could lead to crashes rather than clean shutdown.

バージョン 3.7 で変更: When used with subinterpreters, registered functions are local to the interpreter they were registered in.

atexit.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 register(). It is possible to register the same function and arguments more than once.

この関数は func を返し、これをデコレータとして利用できます。

atexit.unregister(func)

Remove func from the list of exit handlers. 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 atexit call stack will be removed. Equality comparisons (==) are used internally during unregistration, so function references do not need to have matching identities.

参考

readline モジュール

Useful example of atexit to read and write readline history files.

atexit Example

次の簡単な例では、あるモジュールを import した時にカウンタを初期化しておき、プログラムが終了するときにアプリケーションがこのモジュールを明示的に呼び出さなくてもカウンタが更新されるようにする方法を示しています。

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)

register() に指定した位置引数とキーワード引数は登録した関数を呼び出す際に渡されます:

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')

デコレータ として利用する例:

import atexit

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

デコレータとして利用できるのは、その関数が引数なしで呼び出された場合に限られます。