[Patches] [ python-Patches-1602128 ] clarify comparison return values

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Fri Nov 24 08:24:12 CET 2006


Patches item #1602128, was opened at 2006-11-24 02:24
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Category: Documentation
Group: Python 2.4
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Jim Jewett (jimjjewett)
Assigned to: Nobody/Anonymous (nobody)
Summary: clarify comparison return values

Initial Comment:
In the Language Reference section 3.4.1 (http://docs.python.org/ref/customization.html for version 2.5), in the first paragraph where it describes __lt__, __le__, __eq__, __ne__, __gt__, and __ge__,

It should mention the special case of NotImplemented, so that people will not be reluctant to return it.  (The fourth paragraph suggests returning NotImplemented, but the first suggests that *any* return value will be converted to a boolean, so that NotImplented would mean "True")

Please change:

"""These methods can return any value, but if the comparison operator is used in a Boolean context, the return value should be interpretable as a Boolean value, else a TypeError will be raised. By convention, False is used for false and True for true."""

to:

"""When an object does not know how to compute a meaningful result, it should return the singleton  NotImplemented, in case the other object implements the reflected comparison.

By convention, a successful comparison should return either True or False.  If any other object is returned in a Boolean context, python will implicitly call bool(result).
"""

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