SQLAlchemy has its own frozendict which we've had in use for many years, we have it as a pretty well performing cython implementation these days, and I use it ubiquitously. Would be a very welcome addition to the stdlib.

This proposal is important enough that I chimed in on the thread with a detailed example of how SQLAlchemy uses this pattern:

https://discuss.python.org/t/pep-814-add-frozendict-built-in...