atexit.register does not return the registered function.

[pierrerouleau]

BPO	1597824
Nosy	@birkenfeld

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/birkenfeld'
closed_at = <Date 2006-11-16.16:52:16.000>
created_at = <Date 2006-11-16.16:23:31.000>
labels = ['library']
title = 'atexit.register does not return the registered function. '
updated_at = <Date 2006-11-16.16:52:16.000>
user = 'https://bugs.python.org/pierrerouleau'

bugs.python.org fields:

activity = <Date 2006-11-16.16:52:16.000>
actor = 'georg.brandl'
assignee = 'georg.brandl'
closed = True
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2006-11-16.16:23:31.000>
creator = 'pierre_rouleau'
dependencies = []
files = []
hgrepos = []
issue_num = 1597824
keywords = []
message_count = 2.0
messages = ['30564', '30565']
nosy_count = 2.0
nosy_names = ['georg.brandl', 'pierre_rouleau']
pr_nums = []
priority = 'normal'
resolution = 'accepted'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue1597824'
versions = ['Python 2.5']

[pierrerouleau]

Since that the decorator syntax is upon us, I think it would be good if atexit.register() was returning the function passed as argument. This simple change to the library would solve a problem with the use of atexit.register as a decorator (and I can't think of any use case where this change would break any code).

I describe the problem in the following text::

Problem using atexit.register as a decorator
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

In his April 2005 article titled Python 2.4 Decorators: Reducing code duplication and consolidating knowledge_ , Phillip Eby describes how you can use
atexit.register()_ from the standard Python library.
He shows how to use the decorator syntax to register a
function that will execute at program termination. Here
is how it goes::

  @atexit.register
  def goodbye(): 
      print "Goodbye, terminating..."

However, there is one fundamental problem with this:
atexit.register() returns None. Since the above code
corresponds to::

  def goodbye(): 
      print "Goodbye, terminating..."
  goodbye = atexit.register(goodbye)

the code registers goodbye but right after it binds
goodbye to None! You can see this in the following
session::

    >>> import atexit
    >>> @atexit.register
    ... def goodbye():
    ...   print "Goodbye, terminating..."
    ... 
    >>> goodbye()
    Traceback (most recent call last):
      File "<stdin>", line 1, in ?
    TypeError: 'NoneType' object is not callable
    >>> 
    >>> goodbye
    >>> type(goodbye)
    <type 'NoneType'>
    >>> 

There is two solutions to this problem:

1. Use another function to register and decorate.
2. Change atexit.register() in the Python library
   so that it returns the function it registers.

Solution 1 can be implemented right away::

  def atexit_register(fct):
      atexit.register(fct)
      return fct

  @atexit_register
  def goodbye2(): 
      print "Goodbye 2!!"

and it works: it registers the function for execution
at termination but leaves goodbye2 callable::

  >>> def atexit_register(fct):
  ...   atexit.register(fct)
  ...   return fct
  ... 
  >>> @atexit_register
  ... def goodbye2():
  ...   print "Goodbye 2!!"
  ... 
  >>> goodbye2()
  Goodbye 2!!
  >>> goodbye2
  <function goodbye2 at 0x009DD930>
  >>> 

.. References

.. _atexit.register(): http://www.python.org/doc/current/lib/module-atexit.html
.. _Python 2.4 Decorators\: Reducing code duplication and consolidating knowledge: http://www.ddj.com/184406073

[birkenfeld]

This is a reasonable request, I changed atexit.register in rev. 52764 for 2.6.