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To: python-list@python.org
From: Christian Heimes <christian@python.org>
Subject: Re: Differences of "!=" operator behavior in python3 and python2 [ bug? ]
Date: Mon, 13 May 2013 01:39:03 +0200
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Xref: csiph.com comp.lang.python:45212

Am 13.05.2013 01:23, schrieb Mr. Joe:
> I seem to stumble upon a situation where "!=" operator misbehaves in
> python2.x. Not sure if it's my misunderstanding or a bug in python
> implementation. Here's a demo code to reproduce the behavior -
> """

Python 2.7 doesn't use the negation of __eq__ when your class doesn't
provide a __ne__ function. Just add a print() to your __eq__ method and
you'll notice the different.

You have to provide both:


class DemoClass(object):
    def __init__(self, val):
        self.val = val

    def __eq__(self, other):
        if not isinstance(other, DemoClass):
            return NotImplemented
        return self.val == other.val

    def __ne__(self, other):
        if not isinstance(other, DemoClass):
            return NotImplemented
        return self.val != other.val

or

    def __ne__(self, other):
        result = self.__eq__(other)
        if result is NotImplemented:
             return NotImplemented
        return not result