With "del m" you're asking python to delete the whole "m", including "Movie.rating.data" (as "rating.data" is an attribute of "m" that you've just asked to be d

With "del m" you're asking python to delete the whole "m", including "Movie.rating.data" (as "rating.data" is an attribute of "m" that you've just asked to be deleted). But your question isn't 100% clear to me...