* 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.