You may be thinking of the `frozenset()` built in or the third party Python module [frozendict](https://pypi.org/project/frozendict/)?

Personally, I’ve been using a wrapper around `collections.namedtuple` as an underlying data structure to create frozen dictionaries when I’ve needed something like that for a project.

There was a previous PEP (in 2012) with the exact same title:

https://peps.python.org/pep-0416/

Also one in 2019 for a "frozenmap":

https://peps.python.org/pep-0603/

Perhaps you used the frozen dict implementation from the pip installable boltons library: https://boltons.readthedocs.io/en/latest/dictutils.html#bolt...

Perhaps immutabledict?

https://pypi.org/project/immutabledict/