`--pyi_out` does not annotate `__slots__`

[remko]

What version of protobuf and what language are you using?

Language: Python
protobuf 4.21.12
grpcio 1.51.1, grpcio-tools 1.51.1,
Mypy 0.991

What operating system (Linux, Windows, ...) and version?

macOS Ventura

What runtime / compiler are you using (e.g., python version or gcc version)

Python 3.9.16

What did you do?

Run python3 -m grpc_tools.protoc -pyi_out=...

What did you expect to see

A .pyi that validates Mypy type checks.

What did you see instead?

A .pyi that has the following error:

error: Need type annotation for "__slots__"

It seems __slots__ is the only untyped field in the .pyi.

[deannagarcia]

I think this is a bug in Mypy (although the bug suggests the issue has been fixed).

[aiven-anton]

@deannagarcia Changing to using a tuple for __slots__ makes this error go away, which seems reasonable because tuples are immutable. Would you accept a PR that changes all generated __slots__ to use tuples for the pyi generator?

[smheidrich]

I was going to ask to reopen because this is still broken, with protoc outputting lists instead of tuples as mentioned by @aiven-anton. But actually, Python's docs on __slots__ do say

[...] a string, iterable, or sequence of strings [...]

So protoc is not doing anything wrong, lists should be fine and this is just another Mypy bug: python/mypy#10870

Maybe those affected can upvote there?