Chris Angelico <ros...@gmail.com>: > On Thu, Nov 26, 2015 at 5:52 PM, Marko Rauhamaa <ma...@pacujo.net> wrote: >> Nothing prevents using mutable objects as keys in Python. > > Sure, you _can_. But if the key's hash changes between dict insertion > and retrieval, all manner of invariants will break, and likewise if > two equal objects have different hashes. From which you can deduce > logically that any object used as a key must remain (not) equal to > everything that it was (not) equal to from that time until it is > looked up... which basically means its value mustn't change. It must > be immutable.
What I'm saying is that Python does not prevent mutable keys but tries to do that with lists and tuples. I think Python should stop trying. I have wanted to use lists as keys, and there should be no reason to allow mutable tuples. It should be enough to say that the behavior of a dictionary is undefined if a key should mutate on the fly. Marko -- https://mail.python.org/mailman/listinfo/python-list
