1:mod:`atexit` --- Exit handlers
2===============================
3
4.. module:: atexit
5   :synopsis: Register and execute cleanup functions.
6
7.. moduleauthor:: Skip Montanaro <[email protected]>
8.. sectionauthor:: Skip Montanaro <[email protected]>
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**Note:** The effect of registering or unregistering functions from within
24a cleanup function is undefined.
25
26.. versionchanged:: 3.7
27    When used with C-API subinterpreters, registered functions
28    are local to the interpreter they were registered in.
29
30.. function:: register(func, *args, **kwargs)
31
32   Register *func* as a function to be executed at termination.  Any optional
33   arguments that are to be passed to *func* must be passed as arguments to
34   :func:`register`.  It is possible to register the same function and arguments
35   more than once.
36
37   At normal program termination (for instance, if :func:`sys.exit` is called or
38   the main module's execution completes), all functions registered are called in
39   last in, first out order.  The assumption is that lower level modules will
40   normally be imported before higher level modules and thus must be cleaned up
41   later.
42
43   If an exception is raised during execution of the exit handlers, a traceback is
44   printed (unless :exc:`SystemExit` is raised) and the exception information is
45   saved.  After all exit handlers have had a chance to run, the last exception to
46   be raised is re-raised.
47
48   This function returns *func*, which makes it possible to use it as a
49   decorator.
50
51
52.. function:: unregister(func)
53
54   Remove *func* from the list of functions to be run at interpreter shutdown.
55   :func:`unregister` silently does nothing if *func* was not previously
56   registered.  If *func* has been registered more than once, every occurrence
57   of that function in the :mod:`atexit` call stack will be removed.  Equality
58   comparisons (``==``) are used internally during unregistration, so function
59   references do not need to have matching identities.
60
61
62.. seealso::
63
64   Module :mod:`readline`
65      Useful example of :mod:`atexit` to read and write :mod:`readline` history
66      files.
67
68
69.. _atexit-example:
70
71:mod:`atexit` Example
72---------------------
73
74The following simple example demonstrates how a module can initialize a counter
75from a file when it is imported and save the counter's updated value
76automatically when the program terminates without relying on the application
77making an explicit call into this module at termination. ::
78
79   try:
80       with open('counterfile') as infile:
81           _count = int(infile.read())
82   except FileNotFoundError:
83       _count = 0
84
85   def incrcounter(n):
86       global _count
87       _count = _count + n
88
89   def savecounter():
90       with open('counterfile', 'w') as outfile:
91           outfile.write('%d' % _count)
92
93   import atexit
94
95   atexit.register(savecounter)
96
97Positional and keyword arguments may also be passed to :func:`register` to be
98passed along to the registered function when it is called::
99
100   def goodbye(name, adjective):
101       print('Goodbye %s, it was %s to meet you.' % (name, adjective))
102
103   import atexit
104
105   atexit.register(goodbye, 'Donny', 'nice')
106   # or:
107   atexit.register(goodbye, adjective='nice', name='Donny')
108
109Usage as a :term:`decorator`::
110
111   import atexit
112
113   @atexit.register
114   def goodbye():
115       print('You are now leaving the Python sector.')
116
117This only works with functions that can be called without arguments.
118