Abstract
PEP: | 336 |
---|---|
Title: | Make None Callable |
Version: | $Revision$ |
Last-Modified: | $Date$ |
Author: | Andrew McClelland <eternalsquire at comcast.net> |
Status: | Rejected |
Type: | Standards Track |
Content-Type: | text/x-rst |
Created: | 28-Oct-2004 |
Post-History: |
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 [2].
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/reference/ |
[2] | 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.