>>> DummyDict = TypedDict('DummyDict', {'x': int, 'y': str}, total=False)
>>> DummyDict.__required_keys__
frozenset({'x', 'y'})

This happens because the TypedDict function does not pass the "total" metaclass argument to _TypedDictMeta() (instead passing "__total__" in the attribute namespace) and the new code that sets __required_keys__ and __optional_keys__ only checks the metaclass argument.

Can you submit a PR to fix this? It looks like you already have a good understanding of the root cause of the problem, so it should be easy to fix. (Be sure to add a unit test.)

It can be backported to earlier Python versions that have typing.TypedDict (3.8+).

Whoops, sorry, didn't see that you already have a PR. I'll review it next week during the core sprint.

Looking for another reviewer.

New changeset 67b769f5157c9dad1c7dd6b24e067b9fdab5b35d by Alex Grönholm in branch 'master':
bpo-42059: Fix required/optional keys for TypedDict(..., total=False) (GH-22736)
https://github.com/python/cpython/commit/67b769f5157c9dad1c7dd6b24e067b9fdab5b35d

New changeset dbb00062dc3afb12c41c87564e6faefe60766b01 by Miss Islington (bot) in branch '3.9':
bpo-42059: Fix required/optional keys for TypedDict(..., total=False) (GH-22736) (GH-23747)
https://github.com/python/cpython/commit/dbb00062dc3afb12c41c87564e6faefe60766b01

We're looking for a volunteer to port this code to typing_extensions:
https://github.com/python/typing/issues/761

I have backported this to typing_extensions in https://github.com/python/typing/pull/778

On Python 3.9.2 and above typing.TypedDict is used, otherwise it's typing_extensions.TypedDict.

Dominic, thanks so much for the backport!