Proposed new feature or change:

Currently, if any parent class in an inheritance chain does not define __slots__, then all subclasses allow dynamic attribute creation. Even if every subsequent subclass defines __slots__, attributes are still able to be added to instances.

It would be really nice if the NDArrayOperatorsMixin class defined __slots__ = (). This would allow subclasses to prevent dynamic attribute creation by also using __slots__. I don't think there is a downside to adding this and the upside is that subclasses could then choose to add slots of their own and prevent dynamic attribute creation.