Your exact point that for sets, union operator is commutative while for dictionaries it wouldn't be is one of the main objections I foresaw if I ever wrote this up as a PEP. Counterpoint: for numbers, + is commutative but for lists it's not, so it's normal for the same operator to have different commutativity depending on the type of the operands. IMO it's worth using the set operators because of the subclass relation of sets and dicts despite some small (but really, predictable) changes in behavior, but I can see how someone could have a different opinion.