That's going to require quite a lot of changes, it's a giant breaking change. All classes would need someone to go around finding all the attributes that are cr

That's going to require quite a lot of changes, it's a giant breaking change. All classes would need someone to go around finding all the attributes that are created and adding an __slots__ dictionary, to avoid regular attribute initialization in __init__ failing. It's a massive task, and it would completely break backwards compatibility for performance gains that not everybody will need.