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`dict` subclasses can be hashable - it's a very bad idea, but not guarded against. If you were to do this, I'd suggest going the exception way, à la StopIteration.
I wonder how feasible it would be for e.g. dict.__setitem__ to check if a key already exists; it would be a special path that's not taken normally, but filling the mapping on call enables it, and it raises if it sees a duplicate. I think that inserting the check in there would reduce complexity and probably have a negligible impact on performance. I don't know if that's a good idea, just throwing this out here.