Why is there a not equal operator in python [duplicate]

Question

I was wondering about the reason of having a not equal operator in python.

The following snipped:

class Foo:
    def __eq__(self, other):
        print('Equal called')
        return True

    def __ne__(self, other):
        print('Not equal called')
        return True

if __name__ == '__main__':
    a = Foo()

    print(a == 1)
    print(a != 1)
    print(not a == 1)

outputs:

Equal called
True
Not equal called
True
Equal called
False

Doesn't this actually invite a lot of trouble by potentially saying:

A == B and A != B

can be correct at the same time. Furthermore this introduces a potential pitfall when forgetting to implement __ne__.

Answer 1

Depending on one's needs there are cases where equal and not equal are not opposite; however, the vast majority of cases they are opposite, so in Python 3 if you do not specify a __ne__ method Python will invert the __eq__ method for you.

If you are writing code to run on both Python 2 and Python 3, then you should define both.

Answer 2

Per the data model documentation, which covers the "magic methods" you can implement on classes (emphasis mine):

There are no implied relationships among the comparison operators. The truth of x==y does not imply that x!=y is false. Accordingly, when defining __eq__(), one should also define __ne__() so that the operators will behave as expected.