130 lines
2.7 KiB
Plaintext
130 lines
2.7 KiB
Plaintext
PEP: 336
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Title: Make None Callable
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Version: $Revision$
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Last-Modified: $Date$
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Author: Andrew McClelland <eternalsquire@comcast.net>
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Status: Rejected
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Type: Standards Track
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Content-Type: text/x-rst
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Created: 28-Oct-2004
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Post-History:
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Abstract
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========
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``None`` should be a callable object that when called with any
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arguments has no side effect and returns ``None``.
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BDFL Pronouncement
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==================
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This PEP is rejected. It is considered a feature that ``None`` raises
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an error when called. The proposal falls short in tests for
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obviousness, clarity, explicitness, and necessity. The provided Switch
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example is nice but easily handled by a simple lambda definition.
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See python-dev discussion on 17 June 2005 [1]_.
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Motivation
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==========
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To allow a programming style for selectable actions that is more
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in accordance with the minimalistic functional programming goals
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of the Python language.
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Rationale
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=========
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Allow the use of ``None`` in method tables as a universal no effect
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rather than either (1) checking a method table entry against ``None``
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before calling, or (2) writing a local no effect method with
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arguments similar to other functions in the table.
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The semantics would be effectively::
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class None:
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def __call__(self, *args):
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pass
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How To Use
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==========
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Before, checking function table entry against ``None``::
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class Select:
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def a(self, input):
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print 'a'
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def b(self, input):
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print 'b'
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def c(self, input):
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print 'c'
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def __call__(self, input):
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function = { 1 : self.a,
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2 : self.b,
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3 : self.c
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}.get(input, None)
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if function: return function(input)
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Before, using a local no effect method::
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class Select:
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def a(self, input):
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print 'a'
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def b(self, input):
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print 'b'
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def c(self, input):
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print 'c'
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def nop(self, input):
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pass
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def __call__(self, input):
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return { 1 : self.a,
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2 : self.b,
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3 : self.c
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}.get(input, self.nop)(input)
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After::
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class Select:
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def a(self, input):
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print 'a'
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def b(self, input):
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print 'b'
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def c(self, input):
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print 'c'
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def __call__(self, input):
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return { 1 : self.a,
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2 : self.b,
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3 : self.c
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}.get(input, None)(input)
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References
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==========
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.. [1] Raymond Hettinger, Propose to reject PEP 336 -- Make None Callable
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https://mail.python.org/pipermail/python-dev/2005-June/054280.html
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Copyright
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=========
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This document has been placed in the public domain.
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