PEP 814: Add frozendict built-in type

[jamesdow21 (James Dow)]

Couldn’t be happier to see this PEP this morning! Half of the magic wand wish I made in the PEP 603 discussion thread could come true!

@jamesdow21

Overall, my own ideal scenario if I had a magic wand and decision rights would be that both a PEP 416 style frozendict was added as a builtin and a PEP 603 style HAMT based immutable not-a-dict mapping was added to collections.

@jamesdow21

Ideally it could reuse all of the implementation and optimizations that have gone into making dict efficient, keep all the same semantics as dict (e.g. O(1) lookup time, remembering insertion order, equality based on unordered keys and values), but just truly forbid anything that mutates it and add hashing support similar to how tuples work (i.e. hashable if all the values are hashable, raising a TypeError otherwise), then it seems like it would be a useful addition in some cases.

I personally don’t use frozensets very often, but there have been a couple of times where it’s fit a need perfectly and I’ve been very happy to have it, and I imagine that frozendicts would end up the the same