atexit --- 退出处理器

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: 在清理函数内部注册或注销函数可能产生的影响是未定义的。

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.

在 3.12 版本发生变更: 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.

在 3.7 版本发生变更: 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.

这个函数返回 func 对象，可以把它当作装饰器使用。

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.

参见

模块 readline

使用 atexit 读写 readline 历史文件的有用的例子。

atexit 示例

以下简单例子演示了一个模块在被导入时如何从文件初始化一个计数器，并在程序终结时自动保存计数器的更新值，此操作不依赖于应用在终结时对此模块进行显式调用。

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)

位置和关键字参数也可传入 register() 以便传递给被调用的已注册函数:

def goodbye(name, adjective):
    print('Goodbye %s, it was %s to meet you.' % (name, adjective))

import atexit

atexit.register(goodbye, 'Donny', 'nice')
# 或者：
atexit.register(goodbye, adjective='nice', name='Donny')

作为 decorator 使用:

import atexit

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

只有在函数不需要任何参数调用时才能工作。