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* Dicts are not like sets because the ordering operators (<, <=, >, >=) are not defined on dicts, but they implement subset comparisons for sets. I think this is another argument pleading against | as the operator to combine two dicts. * Regarding how to construct the new set in __add__, I now think this should be done like this: class dict: <other methods> def __add__(self, other): <checks that other makes sense, else return NotImplemented> new = self.copy() # A subclass may or may not choose to override new.update(other) return new AFAICT this will give the expected result for defaultdict -- it keeps the default factory from the left operand (i.e., self). * Regarding how often this is needed, we know that this is proposed and discussed at length every few years, so I think this will fill a real need. * Regarding possible anti-patterns that this might encourage, I'm not aware of problems around list + list, so this seems an unwarranted worry to me. -- --Guido van Rossum (python.org/~guido)