This is a Python specific restriction. Ruby for example allows mutable keys in dictionaries.
(0) Object identities are used as keys? (e.g., two different list objects that contain the elements 1,2,3 are treated as different keys)
(1) Catastr... I mean, hilarious things happen if you mutate an object that's being used as a key?
Anyway, both languages use value-based hashing for dictionaries. I am not a Rubyist, but I guess you are just expected to not mutate the keys? Ruby's dictionary implementation also has a rehash method to deal with this case [1], if for some reason you are purposely mutating the keys I suppose...
[1] http://docs.ruby-lang.org/en/2.0.0/Hash.html#method-i-rehash
Ruby, since you said it allows mutable keys in dictionaries.
> Anyway, both languages use value-based hashing for dictionaries.
Neither Python nor Ruby has compound values. Object identities are primitive values.
> but I guess you are just expected to not mutate the keys?
I guess C programmers are expected not to use memory after they free it.