class method __instancecheck__ does not work

Question

I am using python 2.7.3 on Windows. I tried to override the __instancecheck__ magic method as a class method. But I can not make it work.

class          


        

Answer 1

instancecheck must be defined in a metaclass:

class Enumeration(type):
    def __instancecheck__(self, other):
        print 'hi'
        return True


class EnumInt(int):
    __metaclass__ = Enumeration

print isinstance('foo', EnumInt) # prints True

Why is that? For the same reason why your second example worked. When python evaluates isinstance(A, B) it assumes B to be an object, looks for its class and calls __instancecheck__ on that class:

isinstance(A, B):
    C = class-of(B)
    return C.__instancecheck__(A)

But when B is a class itself, then its class C should be a class of a class, in other words, a meta-class!