How can I override class attribute access in python?

Question

How can I override class attribute access in python?

P.S. Is there a way to leave regular access to class attributes alone but calling a more specific exception on missing attribute?

Answer 1

The __getattr__ magic method is called when the attribute doesn't exist on the instance / class / parent classes. You'd use it to raise a special exception for a missing attribute:

class Foo:
    def __getattr__(self, attr):
        # only called when self.attr doesn't exist
        raise MyCustonException(attr)

If you want to customize access to class attributes, you need to define __getattr__ on the metaclass / type:

class BooType(type):
    def __getattr__(self, attr):
        print attr
        return attr

class Boo(metaclass=BooType):
    pass

boo = Boo()
Boo.asd # prints asd
boo.asd # raises an AttributeError like normal

If you want to customize all attribute access, use the __getattribute__ magic method.

Answer 2

agf's answer is correct, and in fact the answer I'm about to give is based on it and owes it some credit.

Just to give more detail on the class-attribute side, which is the part that's actually under question here (as opposed to the difference between __getattr__ and __getattribute__), I'd like to add a reference to an answer I wrote to a different similar question, which goes into more detail on the difference between class and instance attributes, how this is implemented, and why you need to use a metaclass to influence class attribute lookup.