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Both arguments are true and definitive. Last possibility would be to introduce a factory function for defaultdicts that would only accept concrete values:
 from collections import fallbackdict
Then this factory could produce defaultdict instances like this:
 fallbackdict(0, some_key=1) --> defaultdict(lambda: 0, some_key=1)
That would be safe for future type changes as well.
If this version does not sound appealing, I can't see any other way of reliably introducing this shortcut.