Try to extend a class by adding / overriding original method in Python

extend.py

class Foo(object):
    def bar(self):
        return 'bar'


def new_bar(self, super_method):
    if super_method is not None:
        return ', '.join([super_method(), 'new_bar'])
    return 'new_bar'


def extend(cls, method_name, method):
    super_method = getattr(cls, method_name, None)
    def new_method(self, *args, **kwargs):
        if super_method is not None:
            bounded_super = super_method.__get__(self, cls)
        return method(self, bounded_super, *args, **kwargs)
    setattr(cls, method_name, new_method)


old_foo = Foo()
old_foo.bar()  # 'bar'

extend(Foo, 'bar', new_bar)

new_foo = Foo()
new_foo.bar()  # 'bar, new_bar'
old_foo.bar()  # 'bar, new_bar'