Overriding __setattr__ at runtime

Question

I am trying to override the __setattr__ method of a Python class, since I want to call another function each time an instance attribute changes its value. Howev

Answer 1

@SingleNegationElimination's answer is great, but it cannot work with inheritence, since the child class's __mro__ store's the original class of super class. Inspired by his answer, with little change,

The idea is simple, switch __setattr__ before __init__, and restore it back after __init__ completed.

class CleanSetAttrMeta(type):
    def __call__(cls, *args, **kwargs):
        real_setattr = cls.__setattr__
        cls.__setattr__ = object.__setattr__
        self = super(CleanSetAttrMeta, cls).__call__(*args, **kwargs)
        cls.__setattr__ = real_setattr
        return self


class Foo(object):
    __metaclass__ = CleanSetAttrMeta

    def __init__(self):
        super(Foo, self).__init__()
        self.a = 1
        self.b = 2

    def __setattr__(self, key, value):
        print 'after __init__', self.b
        super(Foo, self).__setattr__(key, value)


class Bar(Foo):
    def __init__(self):
        super(Bar, self).__init__()
        self.c = 3

>>> f = Foo()
>>> f.a = 10
after __init__ 2
>>>
>>> b = Bar()
>>> b.c = 30
after __init__ 2