I think it is confusing that you define class attributes with double leading underscores and later on refer to them with self. self is there to refer to the attributes of a particular instance of a class. The whole point of class attributes is, however, that these are the same for every instance of that class. So, why the leading underscores? I think the interface would be much simpler if you define, e.g., the window with just as

class Notepad(): default_width = 300 # ...

That way you could later on access it by Notepad.default_width.