Fix type checking failures

[sirosen]

I recently tried to fix our usage of mypy to get it out of .pre-commit-config.yaml.
The result revealed that type checking really isn't passing -- it's not clear what the hook is doing, but it's not complete type checking.

I moved mypy into a tox environment, created pinned dependencies to mirror the current hook config, and ran it in a relatively naive configuration. mypy failed with 111 errors.

For anyone who wants to reproduce, here's what I tried as a first pass at this in tox.ini:

[testenv:typing]
description = type check the code
deps =
    mypy
    -c constraints/typing.txt
commands =
    mypy --python-version 3.8 piptools/ {posargs}
    mypy --python-version 3.12 piptools/ {posargs}

[testenv:pip-compile-typing]
description = compile constraints for type checking
commands =
    python -m piptools compile --quiet --strip-extras ./pyproject.toml -o ./constraints/pip-tools.txt
    python -m piptools compile --quiet --strip-extras --allow-unsafe ./constraints/typing.in ./constraints/pip-tools.txt -o ./constraints/typing.txt

and constraints/typing.in was:

mypy >= 1.16.1
pip == 20.3.4

---

I'm not sure how we'll want to approach this. I assume the pre-commit hook isn't doing nothing but it's almost definitely not type checking properly.

[webknjaz]

I usually keep MyPy in pre-commit because then it's where all the other linters are, plus https://pre-commit.ci is actually able to run it.

Here's my typical config, let's try to adopt it: https://github.com/sphinx-contrib/sphinxcontrib-towncrier/blob/27bdb93/.pre-commit-config.yaml#L191-L250.

The difference you're seeing must be related to one or more of the following:

1. tests/test_data/ is excluded
2. some deps are pinned differently
3. Python 3.9
4. it passes file paths into MyPy and you pass the entire directory (this used to be most painful for me in the past)

If you look into my config, it's explicit about testing against different Python runtime, and it doesn't pass any paths into the program. Instead, it always runs it with the same context, which is the entire project (kinda).

All the configuration that can possibly be shared lives in the common config @ https://github.com/sphinx-contrib/sphinxcontrib-towncrier/blob/27bdb93/.mypy.ini. This is important because if any other tools were to invoke MyPy outside pre-commit, they'd get the same settings (editor/IDE integrations often do this). The folder+file paths are also stored in that config, along with the overridable defaults (like the Python version).

Actually, here's a stricter example: https://github.com/ansible/awx-plugins/blob/db2cae7/.mypy.ini#L1-L58.

Try to copy that in here when you have a minute, it'll get better. And the pre-commit wrapper is https://github.com/ansible/awx-plugins/blob/db2cae7/tox.ini#L224-L343. It also has some external Python-level strictness + outputs consumable by my reusable-tox.yml that I'd like to eventually bring here.

[sirosen]

Hm. Okay, I can accept that it's possible to run mypy more successfully in pre-commit. My experiences doing this have been pretty uniformly bad. But probably a big part of that is that I haven't had a working configuration to start from, and I see that you're using pass_filenames: false which probably has a big impact.

As an example of the kind of tox config I usually setup, in dependency-groups we have mypy under different Python versions configured: https://github.com/pypa/dependency-groups/blob/29ffcd9c8b7bf692073d6ff245ff2fa364dee581/pyproject.toml#L162-L173
Also, we're trying out the [tool.toml] config over there, which is interesting for a small project like that one.
(I only learned after setting up that config that mypy supports --python-version to control its behavior, so I might do this differently in the future.)