PEP 814: Add frozendict built-in type

[blhsing (Ben Hsing)]

@mikeshardmind

Both of these seem like negatives to implement. Having to check if a frozen type has actually been frozen yet defeats the concurrency safety use case. If we need an efficient builder for it later, the builder shouldn’t be the type itself. I’m sure people can create that without making it a switch on the type itself, or maybe even just an internal fast method when creating from a dict.

Agreed, but since building a dict before shipping it for concurrent consumption is a major use case, I think it makes sense to optimize for the use case by adding a dict.detach method, which creates a frozendict that reuses the hashtable of the dict and make the original dict point to a new empty hashtable, similar to the one for bytearray described in Add zero-copy conversion of `bytearray` to `bytes` by providing `__bytes__()` - #17 by pf_moore.

I also don’t see why you would want dict to inherit from frozendict. Anything that can take either should be able to sufficiently take any Mapping, and checking that instead works better with duck typing and permissive apis.

Agreed too.