[sevensor]

No, I’d type the function argument as a Mapping. Frozendict is so that the function will raise an exception if it violates its type signature.

Edit: that is, if as the caller you want foo to be immutable, then you make it a frozendict

[quietbritishjim]

Ah, I see. The last sentence in your previous comment makes more sense now ("Mapping is great, but ... you can violate it at run time"). A type checker would normally catch violations but I can still see a frozendict would be useful.