Why Do We Get Different Results From Mypy

My colleague added mypy as a test environment for our tox setup.

When I checked out his branch and ran it on my machine, it failed.

The error does not matter, but fwiw it was about that one upstream package has forgotten to include a py.typed file, a topic which I dedicated a whole blog post.

The interesting part was… why did it work on his machine, but not on mine?

I compared the output of both our tox -rvv -e mypy runs… Almost identical, except for the path and a difference in the patch version of Python.

A couple of minutes later my colleague came up with the solution.

Mypy accessed its cache from earlier runs.

Once he deleted the cache, it worked.

In order to prevent future problems, especially which do not resolve when running tox -r, he updated the tox configuration to put the cache directory inside the tox environment so it gets deleted on recreation.

commands =
    mypy --cache-dir="{envdir}/mypy_cache" --strict {posargs:lpcraft}