"atexit" --- 終了ハンドラー
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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。
