Dynamically assign special methods to objects but not classes in Python

Question

I would like to do the following:

class A(object): pass

a = A()
a.__int__ = lambda self: 3

i = int(a)

Unfortunately, this throws:

Answer 1

The only recourse that works for new-style classes is to have a method on the class that calls the attribute on the instance (if it exists):

class A(object):
    def __int__(self):
        if '__int__' in self.__dict__:
            return self.__int__()
        raise ValueError

a = A()
a.__int__ = lambda: 3
int(a)

Note that a.__int__ will not be a method (only functions that are attributes of the class will become methods) so self is not passed implicitly.