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) *Pronouns: he/him **(why is my pronoun here?)* <http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/>
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