PEP: 336
Title: Make None Callable
Version: $Revision$
Last-Modified: $Date$
Author: Andrew McClelland <
[email protected]>
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.
