"atexit" --- 退出处理器
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"atexit" 模块定义了用于注册和取消注册 *退出处理器* 的函数：即在“退出时
”自动执行的函数，也就是说，当程序正常终结（例如，"sys.exit()" 被调用或
主模块执行结束）时或者更一般的情况，当 *interpreter shutdown* 时。

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)

   将 *func* 移出退出处理器列表。 如果 *func* 此前未被注册则
   "unregister()" 将静默地不做任何事。 如果 *func* 已被注册一次以上，
   则该函数每次在 "atexit" 调用栈中的出现都将被移除。 当取消注册时会在
   内部使用相等性比较 ("==")，因此函数引用不需要具有匹配的标识号。

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。
