> Thoughts? (apart from "ugh! it's ugly!" yes I know - it's late here)

Is it guaranteed that no usage pattern can render this protection
inefficient? What if a dict is constructed by intermingling lookups and
inserts?
Similarly, what happens with e.g. the common use case of
dictdefault(list), where you append() after the lookup/insert? Does some
key distribution allow the attack while circumventing the protection?