mypy in ci vs pre-commit

[raybellwaves]

We would like to add typing in xskillscore. @blackary made a PR to add it in using pre-commit (xarray-contrib/xskillscore#320).

For the most part we stay close to xarray for our codebase.

I would like to understand #5110 (comment) a bit more. i.e. what was the decision to bring it out of pre-commit and in to a separate step in gha https://github.com/pydata/xarray/blob/master/.github/workflows/ci-additional.yaml#L149

[keewis]

That's because pre-commit will create a new environment (using venv / virtualenv), which contains only mypy. This means that mypy can check for internal consistency, but the type hints of dependencies (e.g. numpy) won't be used to check compatibility with those (we would need a full environment for that).

pre-commit has the additional_dependencies setting which will install additional packages into the environment, but that's another list of dependencies which can easily get out-of-sync (and I at least didn't know about it when we added the separate mypy CI).