add.py

class metaA(type):
    def __new__(cls, n, b, a):
        #a["__add__"] = lambda x,y:4
        obj = type.__new__(cls, n, b, a)
        #obj.__add__ = lambda x,y:4
        return obj
    def __getattr__(self, name):
        if name == "__add__":
            return lambda x,y:4
        raise AttributeError()

    #def __add__(self, a):
    #    return 4

class A(object, metaclass=metaA):
    def __init__(self):
        setattr(self,'__add__',(lambda x,y:5).__get__(self,A))

a = A()
print(a.__add__(a))
# 5
print(type(a).__add__(a,a))
# 4
print(a + a)
# TypeError