And yes in bullseye we could use python 3.9 to run mypy, I don’t think there would be an issue?

Are you sure that a new tox env is automatically run by zuul? There are three zuul jobs used to run different tox environments I think. Of course we could include mypy typechecking in the “linting” environment, if that’s what you mean.

If we include mypy in tox it should run automatically. The only issue is if we want to run the type checker with a more up to date version of Python as it may not be installed on the current runner. Which I really think we should do. Ideally 3.10, but at the very least we should run with 3.9+ because we won’t have to go re-optimise everything when we update to bullseye

Good question, I assume so. https://stackoverflow.com/questions/56532731/check-type-hint-coverage-in-python
There are ways to generate reports, I assume similar to test coverage reports.

You’re right, we should define a coverage target below 100%. Thing is: I don’t know how to measure typing coverage… Can mypy do that?