[python] Use of True, False, and None as return values in Python functions

Use if foo or if not foo. There isn't any need for either == or is for that.

For checking against None, is None and is not None are recommended. This allows you to distinguish it from False (or things that evaluate to False, like "" and []).

Whether get_attr should return None would depend on the context. You might have an attribute where the value is None, and you wouldn't be able to do that. I would interpret None as meaning "unset", and a KeyError would mean the key does not exist in the file.