Martin: I think the primary reason I suggested an empty __slots__ was to avoid people doing dumb stuff like attaching attributes to the singleton, and then getting annoyed if support for them was removed in some future release (e.g. if this ever moved to C for whatever reason), or expecting them to survive pickling, what have you. The memory savings (if any) would be negligible, it was just to avoid future compatibility concerns.

Serhiy: If we add a stub slot, presumably it should be __stub, not __stub__, the former getting the private member name mangling protection while the latter would not (and the latter might give the impression it was some sort of special method/attribute when it was not).