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

The docs say:

Note that these methods are looked up on the type (metaclass) of a class. They cannot be defined as class methods in the actual class. This is consistent with the lookup of special methods that are called on instances, only in this case the instance is itself a class.

http://docs.python.org/2/reference/datamodel.html#customizing-instance-and-subclass-checks

Answer 2

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!