I meant to post more of these, but here’s one for fun:

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

class B(object):
    def __eq__(self, rhs):
        return False

print A() in [B()]
print B() in [A()]

Maybe not quite as surprising once you see the results and think about it, but getting this wrong was the source of some strange bugs in Pyston.