Add a tox environment to run mypy for static type checking

[namurphy]

Blocked by:

• Add mypy configuration file that ignores existing errors on a per-file and per-error basis #15794

This exploratory pull request would add a tox environment for running mypy for static type checking of type hint annotations. It could be run using:

tox -e mypy

This PR is based off of PlasmaPy/PlasmaPy#2431. A similar PR to add mypy to PlasmaPy's CI suite is PlasmaPy/PlasmaPy#2432. I'm not adding mypy to the CI suite in this PR because that would be a much bigger step than adding a tox environment.

This PR would need to go along with a PR to add a mypy.ini file (e.g., #12971 or #15769) that ignores all current errors, since running mypy without it leads to ∼1–2K errors (depending on the settings we choose).

There has been a great discussion about typing in #15170.

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[pllim]

Does this supersede #15794 ?

[namurphy]

Thank you for the question! It's more like this PR depends on #15794 rather than superseding it. These PRs are two of the three steps that we took for adding mypy to PlasmaPy's CI suite:

1. Configure mypy.ini so that running mypy ignores all current errors, which could be done with either of these PRs:

• Add mypy configuration file that ignores existing errors on a per-file and per-error basis #15794
• mypy incremental adoption settings #12971

2. Add a tox environment for running mypy (this PR)
3. Add a GitHub Action that uses the tox environment (akin to Add mypy to continuous integration suite PlasmaPy/PlasmaPy#2432)

I wanted to separate the PRs in case there's a decision to take a different approach than #15794.

[namurphy]

There are some packages that aren't typed themselves, but have created type stub packages that contain type information. The --install-types flag "causes mypy to install known missing stub packages for third-party libraries using pip", which means that mypy would have access to verified type information for packages like pandas, PyYAML, beautifulsoup4, etc. We could also manually add other type stub packages here, or otherwise specify mypy not to follow certain imports.

[namurphy]

A follow-up to this exploratory PR: I mentioned in this post that it's worth considering alternatives to mypy such as pyright.

[nstarman]

@namurphy, I'm in favor of this! mypy and pyright are sufficiently similar we can start with one and switch to the other, should the need arise. Since mypy is considered the baseline, I think it's good to start with it. This would be a good PR to get in for v7, so maintainers test their type hints!