coding style - identity versus equality for None in Python

Question

Various Python guides say to use x is None instead of x == None. Why is that? Equality is used for comparing values, so it seems natural to ask if x has the value None, denoted with == and not is. Can someone explain why is is the preferred form and show an example where the two do not give the same answer?

Thanks.

Answer 1

The reason people use is is because there is no advantage to using ==. It is possible to write objects that compare equal to None, but it is uncommon.

class A(object):
    def __eq__(self, other):
        return True

print A() == None

Output:

True

The is operator is also faster, but I don't consider this fact important.