Add --prefix flag for uv pip install

[jorng]

When performing multi-stage Docker builds, the --prefix flag is very useful to create an isolated install of dependencies that can then be copied to the final stage of a build.

Example with pip:

FROM python AS builder
# Possibly install prerequisites, gcc, etc, that aren't needed in runtime
COPY requirements.txt ./
RUN pip install -r requirements.txt --prefix /install
# Do other installs / builds...

FROM python:slim
COPY --from=builder /install /usr/local
# Other stuff as needed

Related:

• Add -t/--target flag for uv pip install #1517
• Support --user flag from pip #2077

Notably, this may need to be used in association with --system or --python to specify which python executables are pointed to.

[ericmarkmartin]

We have a use for this at Jane Street. I have a first draft patch that seems to work.

Still need to write tests and think about windows though

[zanieb]

Is --target insufficient?

[charliermarsh]

What’s the use-case?

[ericmarkmartin]

Is --target insufficient?

Target dumps everything into the specified dir. Prefix implies a certain Scheme. Concretely, jupyterlab, for instance, includes plugins and such that should go into share, but --target will dump that stuff alongside the source.

From the linked doc:

The “prefix scheme” is useful when you wish to use one Python installation to perform the build/install ..., but install modules into the third-party module directory of a different Python installation (or something that looks like a different Python installation).

[charliermarsh]

What is the workflow though that demands --prefix as opposed to installing into an arbitrary Python environment via --python? --prefix has its own problems -- IIUC, scripts etc. will reference the parent Python interpreter, not the --prefix Python interpreter.

[charliermarsh]

(By the way, I don't think --prefix is too hard to support, and I'm not really against it -- I just want to make sure that it's justified.)

[charliermarsh]

Do you want advice on your draft? My hope is that you can just change Interpreter::layout to return paths joined against self.virtualenv rather than self.scheme.

[charliermarsh]

I put up a PR here with basic support: #4085.

[ericmarkmartin]

Oh wow that was quick. I was about to put mine up but will take a look at this first

[charliermarsh]

Was hoping to save you some trouble, sorry if I stepped on your toes.

[ericmarkmartin]

Haha no worries--thanks for the help!