28.9. "atexit" — Exit handlers
******************************

New in version 2.0.

**Source code:** Lib/atexit.py

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

The "atexit" module defines a single function to register 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.

This is an alternate interface to the functionality provided by the
"sys.exitfunc()" variable.

Note: This module is unlikely to work correctly when used with other
code that sets "sys.exitfunc".  In particular, other core Python
modules are free to use "atexit" without the programmer’s knowledge.
Authors who use "sys.exitfunc" should convert their code to use
"atexit" instead.  The simplest way to convert code that sets
"sys.exitfunc" is to import "atexit" and register the function that
had been bound to "sys.exitfunc".

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.

   Changed in version 2.6: This function now returns *func*, which
   makes it possible to use it as a decorator.

See also:

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


28.9.1. "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:
       _count = int(open("counter").read())
   except IOError:
       _count = 0

   def incrcounter(n):
       global _count
       _count = _count + n

   def savecounter():
       open("counter", "w").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.
