Expose `UV_PROJECT_ENVIRONMENT` as flag to `uv sync`

[maxmynter]

Summary

I've noticed that UV_PROJECT_ENVIRONMENT has no corresponding flag (docs) to specify it via a flag to the command as e.g. uv sync --venv <my_env>.

Is there a reason for it, would you be open to a PR that introduces this and as how involved would you estimate the change?

Perhaps, I could get some time from work to spend on this as we're interested in that feature.

(I made minor contributions to UV about a year ago but may require some handholding).

Context

We're using UV on our slurm clusters usually have a first job that later launches other child or first class jobs. To avoid races on the shared filesystem as well as redundant work, we pre-build the environments of our uv projects running in these jobs. Now we have new nodes with different architectures which sometimes makes using the prebuild envs blow up. We can simply set the environment variable conditionally, but they are propagated to inherting slurm jobs and it would be nicer to also be able to just pass it as a CLI flag.

Maybe there is also a better way to handle x86 / arm mismatches. Happy for pointers if you have ideas.

Cheers and thank you for making Python a better place :)

[zanieb]

Hm. I don't want to add a CLI argument for it because it's intended to be an escape hatch. You have a compelling use-case though.

Have you tried using a .venv symlink instead of setting UV_PROJECT_ENVIRONMENT? We might be breaking that with #18214 though. It's possible centralized storage would solve this though, as we derive a unique key for you.

It seems the ideal situation could be something like #14937 but that's unlikely to land.

[sr-murthy]

It seems UV does not detect the current active virtual environment if it's located outside the working directory, but relies on .venv in the working directory. There are situations where pre-existing virtual environments are located, for various reasons, outside the working directory. Fully understand that UV_PROJECT_ENVIRONMENT can be set, but surely could also be done by amending uv venv to auto-detect? I think PDM some kind of auto-detection that works well, in my experience.

[DhavalGojiya]

It seems UV does not detect the current active virtual environment if it's located outside the working directory,

This is not true.

[sr-murthy]

Really? Well, it didn't work for me, and I had to explicitly set this flag.

[DhavalGojiya]

Really? Well, it didn't work for me, and I had to explicitly set this flag.

% uv sync --active
The --active flag do the work.