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/Doc/library/atexit.rst

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  1
  2:mod:`atexit` --- Exit handlers
  3===============================
  4
  5.. module:: atexit
  6   :synopsis: Register and execute cleanup functions.
  7.. moduleauthor:: Skip Montanaro <skip@pobox.com>
  8.. sectionauthor:: Skip Montanaro <skip@pobox.com>
  9
 10
 11.. versionadded:: 2.0
 12
 13The :mod:`atexit` module defines a single function to register cleanup
 14functions.  Functions thus registered are automatically executed upon normal
 15interpreter termination.
 16
 17Note: the functions registered via this module are not called when the program
 18is killed by a signal, when a Python fatal internal error is detected, or when
 19:func:`os._exit` is called.
 20
 21.. index:: single: exitfunc (in sys)
 22
 23This is an alternate interface to the functionality provided by the
 24``sys.exitfunc`` variable.
 25
 26Note: This module is unlikely to work correctly when used with other code that
 27sets ``sys.exitfunc``.  In particular, other core Python modules are free to use
 28:mod:`atexit` without the programmer's knowledge.  Authors who use
 29``sys.exitfunc`` should convert their code to use :mod:`atexit` instead.  The
 30simplest way to convert code that sets ``sys.exitfunc`` is to import
 31:mod:`atexit` and register the function that had been bound to ``sys.exitfunc``.
 32
 33
 34.. function:: register(func[, *args[, **kargs]])
 35
 36   Register *func* as a function to be executed at termination.  Any optional
 37   arguments that are to be passed to *func* must be passed as arguments to
 38   :func:`register`.
 39
 40   At normal program termination (for instance, if :func:`sys.exit` is called or
 41   the main module's execution completes), all functions registered are called in
 42   last in, first out order.  The assumption is that lower level modules will
 43   normally be imported before higher level modules and thus must be cleaned up
 44   later.
 45
 46   If an exception is raised during execution of the exit handlers, a traceback is
 47   printed (unless :exc:`SystemExit` is raised) and the exception information is
 48   saved.  After all exit handlers have had a chance to run the last exception to
 49   be raised is re-raised.
 50
 51   .. versionchanged:: 2.6
 52      This function now returns *func* which makes it possible to use it as a
 53      decorator without binding the original name to ``None``.
 54
 55
 56.. seealso::
 57
 58   Module :mod:`readline`
 59      Useful example of :mod:`atexit` to read and write :mod:`readline` history files.
 60
 61
 62.. _atexit-example:
 63
 64:mod:`atexit` Example
 65---------------------
 66
 67The following simple example demonstrates how a module can initialize a counter
 68from a file when it is imported and save the counter's updated value
 69automatically when the program terminates without relying on the application
 70making an explicit call into this module at termination. ::
 71
 72   try:
 73       _count = int(open("/tmp/counter").read())
 74   except IOError:
 75       _count = 0
 76
 77   def incrcounter(n):
 78       global _count
 79       _count = _count + n
 80
 81   def savecounter():
 82       open("/tmp/counter", "w").write("%d" % _count)
 83
 84   import atexit
 85   atexit.register(savecounter)
 86
 87Positional and keyword arguments may also be passed to :func:`register` to be
 88passed along to the registered function when it is called::
 89
 90   def goodbye(name, adjective):
 91       print 'Goodbye, %s, it was %s to meet you.' % (name, adjective)
 92
 93   import atexit
 94   atexit.register(goodbye, 'Donny', 'nice')
 95
 96   # or:
 97   atexit.register(goodbye, adjective='nice', name='Donny')
 98
 99Usage as a :term:`decorator`::
100
101   import atexit
102
103   @atexit.register
104   def goodbye():
105       print "You are now leaving the Python sector."
106
107This obviously only works with functions that don't take arguments.
108