1:mod:`atexit` --- Exit handlers
2===============================
3
4.. module:: atexit
5   :synopsis: Register and execute cleanup functions.
6
7.. moduleauthor:: Skip Montanaro <skip@pobox.com>
8.. sectionauthor:: Skip Montanaro <skip@pobox.com>
9
10--------------
11
12The :mod:`atexit` module defines functions to register and unregister cleanup
13functions.  Functions thus registered are automatically executed upon normal
14interpreter termination.  :mod:`atexit` runs these functions in the *reverse*
15order in which they were registered; if you register ``A``, ``B``, and ``C``,
16at interpreter termination time they will be run in the order ``C``, ``B``,
17``A``.
18
19**Note:** The functions registered via this module are not called when the
20program is killed by a signal not handled by Python, when a Python fatal
21internal error is detected, or when :func:`os._exit` is called.
22
23.. versionchanged:: 3.7
24    When used with C-API subinterpreters, registered functions
25    are local to the interpreter they were registered in.
26
27.. function:: register(func, *args, **kwargs)
28
29   Register *func* as a function to be executed at termination.  Any optional
30   arguments that are to be passed to *func* must be passed as arguments to
31   :func:`register`.  It is possible to register the same function and arguments
32   more than once.
33
34   At normal program termination (for instance, if :func:`sys.exit` is called or
35   the main module's execution completes), all functions registered are called in
36   last in, first out order.  The assumption is that lower level modules will
37   normally be imported before higher level modules and thus must be cleaned up
38   later.
39
40   If an exception is raised during execution of the exit handlers, a traceback is
41   printed (unless :exc:`SystemExit` is raised) and the exception information is
42   saved.  After all exit handlers have had a chance to run the last exception to
43   be raised is re-raised.
44
45   This function returns *func*, which makes it possible to use it as a
46   decorator.
47
48
49.. function:: unregister(func)
50
51   Remove *func* from the list of functions to be run at interpreter
52   shutdown.  After calling :func:`unregister`, *func* is guaranteed not to be
53   called when the interpreter shuts down, even if it was registered more than
54   once.  :func:`unregister` silently does nothing if *func* was not previously
55   registered.
56
57
58.. seealso::
59
60   Module :mod:`readline`
61      Useful example of :mod:`atexit` to read and write :mod:`readline` history
62      files.
63
64
65.. _atexit-example:
66
67:mod:`atexit` Example
68---------------------
69
70The following simple example demonstrates how a module can initialize a counter
71from a file when it is imported and save the counter's updated value
72automatically when the program terminates without relying on the application
73making an explicit call into this module at termination. ::
74
75   try:
76       with open("counterfile") as infile:
77           _count = int(infile.read())
78   except FileNotFoundError:
79       _count = 0
80
81   def incrcounter(n):
82       global _count
83       _count = _count + n
84
85   def savecounter():
86       with open("counterfile", "w") as outfile:
87           outfile.write("%d" % _count)
88
89   import atexit
90   atexit.register(savecounter)
91
92Positional and keyword arguments may also be passed to :func:`register` to be
93passed along to the registered function when it is called::
94
95   def goodbye(name, adjective):
96       print('Goodbye, %s, it was %s to meet you.' % (name, adjective))
97
98   import atexit
99   atexit.register(goodbye, 'Donny', 'nice')
100
101   # or:
102   atexit.register(goodbye, adjective='nice', name='Donny')
103
104Usage as a :term:`decorator`::
105
106   import atexit
107
108   @atexit.register
109   def goodbye():
110       print("You are now leaving the Python sector.")
111
112This only works with functions that can be called without arguments.
113