python-peps/pep-0336.txt

PEP: 336
Title: Make None Callable
Version: $Revision$
Last-Modified: $Date$ 
Author: Andrew McClelland
Status: Rejected
Type: Standards Track
Content-Type: text/plain 
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, explictness, and necessity.  The provided Switch
    example is nice but easily handled by a simple lambda definition.
    See python-dev discussion on 17 June 2005.


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] Python Reference Manual, Section 3.2,
        http://docs.python.org/ref/ref.html


Copyright

    This document has been placed in the public domain.



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