Is it ok to add this file even though the whole project isn't typed? Can it be added only to a certain sub package? In that case we could add it when such a package is properly typed. For example, we have #4841 about typing the whole robot.api package.

Notice that typing the whole Robot code page in the near future isn't a realistic goal, but typing all public APIs is something we can try.

Is it ok to add this file even though the whole project isn't typed?

With this file, mypy will see the modules/classes/functions. without, it will only complain.

Can it be added only to a certain sub package?

I'm pretty sure you can.

My understanding of py.typed is that indicates that the module is properly typed. That's not true for the robot module and, very importantly, isn't even a goal in the foreseeable future. We have, however, an issue about properly typing the robot.api module and the plan is to add py.typed there once we are ready.

If you can point me to some authoritative documentation stating that adding py.typed is fine even if types are incomplete, we can add it. Adding it wouldn't make typing the whole code base a goal, though, and we'd preserve the right to remove the file if it causes problems or confusion.

I understand that some users would benefit from more detailed typing. I think the best solution would be such users creating stub files that can be distributed separately.