Re: Differences of "!=" operator behavior in python3 and python2 [ bug? ]

[Ned Batchelder]

On 5/12/2013 7:23 PM, Mr. Joe wrote:
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 -
"""
# -*- coding: utf-8 -*-
from __future__ import unicode_literals, print_function
class DemoClass(object):
     def __init__(self, val):
         self.val = val

     def __eq__(self, other):
         return self.val == other.val

x = DemoClass('a')
y = DemoClass('a')

print("x == y: {0}".format(x == y))
print("x != y: {0}".format(x != y))
print("not x == y: {0}".format(not x == y))
"""

In python3, the output is as expected:
"""
x == y: True
x != y: False
not x == y: False
"""

In Python 3, if __ne__ isn't defined, "!=" will call __eq__ and negate 
the result.
In python2.7.3, the output is:
"""
x == y: True
x != y: True
not x == y: False
"""
Which is not correct!!

In Python 2, "!=" only calls __ne__.  Since you don't have one defined, 
it's using the built-in object comparison, and since x and y are 
different objects, they are not equal to each other, so x != y is True.

Thanks in advance for clarifications.
Regards,
TB

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