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

Question

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 -

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 python2.7.3, the output is:

x == y: True
x != y: True
not x == y: False

which is not correct!!

Answer 1

In Python 3, if __ne__ isn't defined, "!=" will call __eq__ and negate the result.

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.

Answer 2

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

Answer 3

The != operator is implemented by the __ne__ special method. In Python 3, the default implementation of __ne__ is to call __eq__ and return the opposite of whatever it returns. In Python 2, __ne__ calls the older __cmp__ method instead, which is no longer meaningful in Python 3.