[kilink]

This is a Python specific restriction. Ruby for example allows mutable keys in dictionaries.

[catnaroek]

So, which is the case?

(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?

[kilink]

I'm not sure if you are asking about Python, or Ruby.

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

[catnaroek]

> I'm not sure if you are asking about Python, or Ruby.

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.

[kazinator]

But if Python got rid of tuples, it would be a radical change; in that case it could as well allow lists to be dictionary keys in the same stroke.