Discovery of local venv

[MichaReiser]

Red Knot currently requires users to configure the path to their venv explicitly. Red Knot should try to detect the presence of a venv by following community best practices ($VIRTUAL_ENV, .venv, venv).

Related: uv's supported python sources

[AlexWaygood]

Red Knot should try to detect the precense of a venv by following community best practices ($VIRTUAL_ENV, .venv, venv).

The last two of these are sort-of heuristics: virtual environments are also often called .env, env, or something else altogether. I'm okay with .venv, as there have been effors (supported by uv and several other tools) to make .venv a standard name that should be used by all Python virtual environments. But I'd vote for keeping the special-casing to just .venv, and not applying the special-casing to venv. It's a much less common name for the virtual environment, and there's no attempt to make a standard around it.

[MichaReiser]

Let's keep this open because we may want to support picking up .venv even if VIRTUAL_ENV isn't set.

[AlexWaygood]

Let's keep this open because we may want to support picking up .venv even if VIRTUAL_ENV isn't set.

If we do this, do we want to also support a --no-site-packages flag to allow users to avoid this implicit fallback? It might not be welcome in all cases; there are some cases where you want to ensure isolation and don't want the type checker to use any types from the virtual environment, even if there is one

[MichaReiser]

One more thing we should consider here is the LSP integration where VIRTUAL_ENV is unlikely to be set.

[AlexWaygood]

Huh, I always have the project's virtual environment activated when I'm working on a Python project in my IDE (and if the virtual environment is activated, the VIRTUAL_ENV variable is certain to be set). It's impossible to e.g. run tests from a terminal in my IDE if the correct virtual environment isn't activated.

But even if the virtual environment isn't activated, pylance always forces me to explicitly tell it which system installation or virtual environment it should use for the project (from a drop-down menu) before I can start working on a project

[MichaReiser]

Yes, this is a functionality that pylance (the python extension) provides. We should either integrate with the python extension (the downside here is that the python extension comes with its own LSP) or implement our own Python/venv discovery.