"atexit" --- 退出處理函式
*************************

======================================================================

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：**在清理函式中註冊或註銷函式的作用並未定義。

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 版的變更: 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 版的變更: 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.

   該函式回傳 *func*，這使得可以將其作為裝飾器使用。

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.

也參考:

  "readline" 模組
     "atexit" 用於讀取和寫入 "readline" 歷史檔案的一個不錯的範例。


"atexit" 範例
=============

以下的簡單範例示範了模組如何在被引入時以檔案來初始化計數器，並在程式終
止時自動儲存計數器的更新值，而不需要仰賴應用程式在終止時明確呼叫該模組
。

   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)

位置引數和關鍵字引數也可以被傳遞給 "register()"，以便在呼叫時也傳遞給
已註冊函式：

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

   import atexit

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

作為*裝飾器*使用：

   import atexit

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

這只適用於可以不帶引數呼叫的函式。
