A couple of points:

1. Eric's proposal is what I had in mind with the `fallbackdict' idea.
2. I'm also reluctant to add more variants to the standard library. Then again if it contained a `fallbackdict' I wouldn't probably ever use `defaultdict' again. How often do you need to provide a factory?
3. Naming the other variant `defaultdict_const', `defaultdict_value', `defaultdict_whatever' beats the purpose because it's actually more characters to type than `defaultdict(lambda:', especially when you count the longer import name.
4. I cannot come up with another typical integer value that would be useful, then again I've used "", [] and set() numerous times. Adding zerodict, stringdict, listdict, setdict is obviously absurd.
5. The discussion started on Twitter amongst a couple of core devs and __missing__ didn't appear to be the one obvious way to anyone.
6. Of course I'm in no position to reject Guido's design on anything. Then again even `defaultdict(lambda:' is simply so much shorter than subclassing dict.

To sum up: if you don't find the idea of adding `fallbackdict' (possibly with an different *short* name) worth it, then I'm +1 on correcting the docs in terms of __missing__ and leaving the implementation as is.