Automatically detect when uv is being used

[godalming123]

Feature

If .venv/bin/python exists in the root of the project (where the pyproject.toml file is), then automatically use it as the python executable with priority over /usr/bin/python.

Pitch

This feature would make uv work out of the box, instead of a user having to figure out that they need to add --python-executable=.venv/bin/python to the mypy command that they run.

Alternative solutions

There could also be a help note if .venv/bin/python exists and mypy cannot find implementations or library stubs for the majority of the imports, that suggests adding --python-executable=.venv/bin/python to the mypy command.

[sterliakov]

Usually you have mypy installed in the same virtual environment, why are you running system installation of mypy against a codebase in venv? --python-executable exists to support that scenario, but it should be vanishingly rare. Given that mypy changes significantly with every release, relying on system installation is just begging for trouble, different projects may depend on different mypy versions.

[godalming123]

How am I meant to use mypy with uv, since the documentation only explains how to use mypy with pip? Running uvx mypy, still can't find any of the imports to external libraries unless you add --python-executable=.venv/bin/python to the command. I'm happy to update the documentation to add instructions for using mypy with uv.

[brianschubert]

Using uv vs pip (or uvx vs pipx) doesn't actually matter per-se. What matters is whether mypy is running in the same virtual environment as where your project / its dependencies are installed. If you run mypy using uxv mypy, mypy will be installed in an isolated virtual environment that does not include your project's dependencies. If you instead install mypy into your project's virtual enviroment (e.g. with uv venv; uv pip install mypy), things should work as expected.

Also related: #17214

[A5rocks]

Note that uvx will use a tool in the relevant venv if it's installed. You just need to add mypy as a dev dependency for the relevant project.

Nevermind I somehow confused uv tool run (=uvx) and uv run. Worse yet, in JS-land npx has above behavior :/

[hauntsaninja]

I wouldn't want us to automatically use .venv/bin/python. Adding the suggestion as a "note" additional diagnostic on import not found errors similar to #17214 could be good though!