29.8. "atexit" — Exit handlers
******************************

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

The "atexit" module defines functions to register and unregister
cleanup functions.  Functions thus registered are automatically
executed upon normal interpreter termination.  "atexit" runs these
functions in the *reverse* order in which they were registered; if you
register "A", "B", and "C", at interpreter termination time they will
be run in the order "C", "B", "A".

**Note:** The functions registered via this module 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.

atexit.register(func, *args, **kwargs)

   Register *func* as a function to be executed at termination.  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.

   At normal program termination (for instance, if "sys.exit()" is
   called or the main module’s execution completes), all functions
   registered are called in last in, first out order.  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 the exit handlers, 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.

   This function returns *func*, which makes it possible to use it as
   a decorator.

atexit.unregister(func)

   Remove *func* from the list of functions to be run at interpreter
   shutdown.  After calling "unregister()", *func* is guaranteed not
   to be called when the interpreter shuts down, even if it was
   registered more than once.  "unregister()" silently does nothing if
   *func* was not previously registered.

See also:

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


29.8.1. "atexit" Example
========================

The following simple example demonstrates how a module can initialize
a counter from a file when it is imported and save the counter’s
updated value automatically when the program terminates without
relying on the application making an explicit call into this module at
termination.

   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)

Positional and keyword arguments may also be passed to "register()" to
be passed along to the registered function when it is called:

   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')

Usage as a *decorator*:

   import atexit

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

This only works with functions that can be called without arguments.
