I don't understand the obsession with Counter here. IMO it's a prime example of sharing implementation through inheritance, an antipattern. It implements the Mapping (i.e. read-only) interface fine, but while it has all the methods of the MutableMapping interface, the behavior is sufficiently different that you shouldn't pass it to code that was written with a MutableMapping or dict in mind. It would have made more sense to design an API specifically for Counter, and have the implementation use a dict internally to hold the values (the "composition over inheritance" pattern).

--Guido van Rossum (python.org/~guido)