In Python 3 __ne__ by default delegates to __eq__ and inverts the
result, but in Python 2 they urge you to define __ne__ when you define
__eq__ for it to work properly . 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.