Allow sync to install to system python

[nhumrich]

In docker environments, it's quite common to install all python packages to the system. Virtual environments are not needed because the current project is the only thing installed.

The docker documentation for UV recommends that you use the venv entrypoint for all container commands:

CMD ["/app/.venv/bin/python"]

However, this makes migrating to UV a bit harder when so many things already reference "python" directly in.

Also, using a virtual environment in these situations complicates the container, because devs now have to remember to use the .venv python always.

Would it be possible to install into the system during uv sync somehow. I assumed --python-preference only-system would do this, but that doesn't seem to prevent the creating of a virtual environment. uv pip install support --system, so its sort of strange that sync does not.

My current workaround is doing this in the dockerfile instead:

RUN --mount=from=ghcr.io/astral-sh/uv,source=/uv,target=/bin/uv \
    uv pip compile pyproject.toml -o requirements.txt && \
    uv pip install -r requirements.txt --system

[zanieb]

See UV_PROJECT_ENVIRONMENT and #6834

[nhumrich]

@zanieb maybe I am misunderstanding, but it appears as if UV_PROJECT_ENVIRONMENT wants the location of a virtualenv.

The intent is not to tell UV to use an existing virtual environment, but to use the system directly.

The docs also say:

uv sync will remove extraneous packages from the environment by default and, as such, may leave the system in a broken state.

This is not wanted. I would conclude that this feature request does not appear to be the same as the UV_PROJECT_ENVIRONMENT feature.

[zanieb]

uv sync will remove extraneous packages from the environment by default and, as such, may leave the system in a broken state.

You can use --inexact then, but it still might change package versions if it needs to — which is the problem with using a system environment generally. Why are you using uv sync to a system environment if you don't have control over the environment?

[nhumrich]

@zanieb I do have control over the environment. Its a docker container.

[zanieb]

Then why are you worried about packages being removed? Sorry, I must be missing something.