add ability to load from .env files before invoking the command

[haydn-j-evans]

Summary

Pipenv and other equivalent tools have the ability to load an .env file during execution of a script etc

https://pipenv.pypa.io/en/stable/shell.html#automatic-loading-of-env

The PR adds a new flag --load-dotenv and a new ENV UV_RUN_LOAD_DOTENV that enables this feature for uv run

resolves #1384

Test Plan

Have tested it locally by successfully invoking scripts that require ENV's to be set.

[eth3lbert]

dotenv is implicitly unmaintained.
Ref: rustsec/advisory-db#1254

[haydn-j-evans]

@eth3lbert I will switch it to dotenvy if you are happy with the idea of the PR :)

[theintz]

Can't wait for this to land, we're excited to switch to uv, but are currently blocked by this.

[haydn-j-evans]

@charliermarsh This PR is currently configured to be opt in, but in light of the discussion in the issue thread, should we instead make it opt out?

i.e.

--no-load-dotenv

UV_RUN_NO_LOAD_DOTENV

And also the naming can be discussed, I kept it scoped to UV_RUN_ as this is a run specific variable, but perhaps if there is a need for full uv configuration with a .env, we just call it UV_NO_LOAD_DOTENV and then just expand its use within uv later.

[PhilipVinc]

Could this be used to set UV_PROJECT_ENVIRONMENT in the .env file, offering a slightly better workaround to the issues discussed in #7642 and #1495 ?

[haydn-j-evans]

@PhilipVinc currently it only affects the run command, it would have to be further expanded to support uv global config.

[haydn-j-evans]

currently this will just fail silently if the file is malformed or missing - is this what we want?

[sminnee]

IMO missing shouldn't show an error, because the chance that the file is missing means that someone doesn't want to use .env is high.

However, a malformed file should be indicated to the developer. The chance that they have intentionally created a .env file that is malformed, in order for it not to be loaded by uv, is very low. I would suggest a warning rather than an error, because execution can still continue.

[charliermarsh]

Yeah, I think we should show a user-facing warning if there's an error other than "file does not exist". We could also show a warning if it's "file does not exist" and the user provided an explicit path to a dotenv file, but I'm not certain if we should and it's not blocking.

[charliermarsh]

Yeah I think my preference would be:

• Read .env by default (only in uv run for now; we may expand it later).
• Allow users to pass --env-file (or UV_ENV_FILE) to override the file.
• Add --no-env-file and UV_NO_ENV_FILE to disable it.

I think we can omit the UV_RUN_ prefixes.

[haydn-j-evans]

closing this as it seems to be implemented already