python-peps/peps/pep-0336.rst

PEP: 336
Title: Make None Callable
Version: $Revision$
Last-Modified: $Date$
Author: Andrew McClelland <eternalsquire@comcast.net>
Status: Rejected
Type: Standards Track
Content-Type: text/x-rst
Created: 28-Oct-2004
Post-History:


Abstract
========

``None`` should be a callable object that when called with any
arguments has no side effect and returns ``None``.


BDFL Pronouncement
==================

This PEP is rejected.  It is considered a feature that ``None`` raises
an error when called.  The proposal falls short in tests for
obviousness, clarity, explicitness, and necessity.  The provided Switch
example is nice but easily handled by a simple lambda definition.
See python-dev discussion on 17 June 2005 [1]_.


Motivation
==========

To allow a programming style for selectable actions that is more
in accordance with the minimalistic functional programming goals
of the Python language.


Rationale
=========

Allow the use of ``None`` in method tables as a universal no effect
rather than either (1) checking a method table entry against ``None``
before calling, or (2) writing a local no effect method with
arguments similar to other functions in the table.

The semantics would be effectively::

    class None:

        def __call__(self, *args):
            pass


How To Use
==========

Before, checking function table entry against ``None``::

    class Select:

        def a(self, input):
            print 'a'

        def b(self, input):
            print 'b'

        def c(self, input):
            print 'c'

        def __call__(self, input):
         function = { 1 : self.a,
               2 : self.b,
               3 : self.c
            }.get(input, None)
         if function:  return function(input)

Before, using a local no effect method::

    class Select:

        def a(self, input):
            print 'a'

        def b(self, input):
            print 'b'

        def c(self, input):
            print 'c'

        def nop(self, input):
         pass

        def __call__(self, input):
            return { 1 : self.a,
            2 : self.b,
            3 : self.c
            }.get(input, self.nop)(input)

After::

    class Select:

        def a(self, input):
            print 'a'

        def b(self, input):
            print 'b'

        def c(self, input):
            print 'c'

        def __call__(self, input):
         return { 1 : self.a,
            2 : self.b,
            3 : self.c
            }.get(input, None)(input)


References
==========

.. [1] Raymond Hettinger, Propose to reject PEP 336 -- Make None Callable
       https://mail.python.org/pipermail/python-dev/2005-June/054280.html


Copyright
=========

This document has been placed in the public domain.