Feature request: add a way to install only build dependencies

[stefanv]

ref: pypa/pip#11440 (comment)

In our CI, we often need to install build-dependencies for a project, without installing the project itself.
At the moment, there is no easy way of telling pip to do that.

We've resorted to compiling requirements files from pyproject.toml, but it would be convenient if we could install dependencies directly.

[webknjaz]

In our CI, we often need to install build-dependencies for a project, without installing the project itself.
At the moment, there is no easy way of telling pip to do that.

May I suggest making the use case/motivation more detailed? You're jumping to the proposed solution here but it's not clear why you need this. Are you trying to have a manually managed PEP 517 build environment? An example would be helpful FTR.

[stefanv]

Like I wrote above, it's useful for a developer setup or CI where the need is to install dependencies without compiling the project. Think of the case, e.g., where you want to prepare a new environment for repeated builds, using no-build-isolation.

[webknjaz]

Okay, so it's indeed about PEP 517 / 660. I wasn't sure that it's not about PEP 735.
Strictly speaking, the project itself is never installed into the build env in any of the build frontends I know of.

[stefanv]

PEP 735: "This is suitable for creating named groups of dependencies, similar to requirements.txt files." — since we currently use requirements.txt for this purpose, it seems related?

[webknjaz]

it seems

Well, that's what I'm trying to determine by asking to spell out the use case details.

It could be viewed as related but it doesn't directly cover [build-system] due to focusing on a very specific table it defines — [dependency-groups], effectively leaving it out.

Though you raise a valid question — can the PEP 517 build deps be a special-cased dependency group. And if you ask me, I'd say: "It's complicated".
This is because this section specifically targets packageable projects and dependency groups are more generic. Also, PEP 517 build deps aren't defined statically more often than not — for example, you can statically define a dependency on setuptools and that would be your single build dependency for building sdists. But as soon as you start producing wheels, another dependency appears — wheel. setuptools injects it dynamically through the corresponding PEP 517 hook. So special-casing such a dependency would inevitably open up a can of worms.

Still, it might be useful to let the PEP author know so that it could be called out in the document.

[konstin]

@stefanv I've seen you linked scikit-image as an example, could you walk me through what the workflow there (or in another shareable repo) is? I'm trying to follow along what the need here workflow-wise is so we can add something to uv that fills that gap.

I'm also inviting others to share their usage scenarios and pipelines, this would be extremely helpful for us for coming up with the right design and test case.

[webknjaz]

In my last year's rant I enumerated some of the dependency groups that may exist in various workflows: jazzband/pip-tools#1326 (comment). Might be useful FTR.

[stefanv]

@stefanv I've seen you linked scikit-image as an example, could you walk me through what the workflow there (or in another shareable repo) is? I'm trying to follow along what the need here workflow-wise is so we can add something to uv that fills that gap.

Sure thing. We have a pyproject.toml file that declares build and other types of dependencies. When then have a pre-commit hook that ensures that pyproject.toml dependencies are exported to requirements.txt files.

For simplicity, we declare all dependencies as optional package dependencies (including re-declaring build dependencies), but ideally there would be no duplication.

The above configuration allows us to:

1. Have one definitive place to define various types of dependencies.
2. Install those dependencies by themselves, as needed. E.g., in our CI (another example).

Another scenario where this is useful is for developing, say, NumPy. I want to create an environment, install all build dependencies, and then run the standard "in-place" developer build: spin build.

Developer builds require that the environment already has dependencies installed. The only existing way I know of to get those dependencies installed is to install the package using "no build isolation", and to then uninstall it again.

[webknjaz]

Developer builds require that the environment already has dependencies installed. The only existing way I know of to get those dependencies installed is to install the package using "no build isolation", and to then uninstall it again.

Looking into https://github.com/scikit-image/scikit-image/blob/628adaa/tools/generate_requirements.py#L32-L34, it seems like you don't really want build deps in the terms of PEP 517 but runtime deps that happen to be needed too. and the contents of requirements/ confirms that suspicion...
There's nothing in your scripts retrieving https://github.com/scikit-image/scikit-image/blob/628adaa/pyproject.toml#L120-L129. Yet, for some reason I can't fully comprehend, you think that "no build isolation" helps you get those deps. Is is that you want Cython for in-place builds or why?

FWIW we also have something similar for the runtime deps: https://github.com/aio-libs/aiohttp/blob/6c3122f/Makefile#L187-L190 / https://github.com/aio-libs/aiohttp/blob/6c3122f/requirements/sync-direct-runtime-deps.py / https://github.com/aio-libs/aiohttp/blob/6c3122f/requirements/runtime-deps.in. While keeping all the "dependency groups" in separate requirements files that are then processed by pip-compile to produce constraints.

In another project with a focus on maximum CI stability, I do indeed retrieve [build-system].requires that I then resolve into constraints and set to PIP_CONSTRAINT for package install reproducibility: https://github.com/cherrypy/cheroot/blob/0fd16f0/requirements/dist-build-constraints.txt.
There, I made a pip wrapper (https://github.com/cherrypy/cheroot/blob/0fd16f0/bin/pip-wrapper / https://github.com/cherrypy/cheroot/blob/0fd16f0/bin/pip_constraint_helpers.py) integrated into tox (https://github.com/cherrypy/cheroot/blob/0fd16f0/tox.ini#L22-L29) that auto-selects a proper constraint file from the matrix for the current env and auto-injects it into the pip install command via -c. Dependency groups for the envs involved are hand-crafted (https://github.com/cherrypy/cheroot/tree/0fd16f0/requirements) with a subset of the constraints being auto-generated/updated via GHA that provides access to many different runtimes (https://github.com/cherrypy/cheroot/blob/0fd16f0/.github/workflows/pip-tools.yml#L51-L66), which addresses the problem with inability to cross-compile transitive deps that need to be built from source.

[stefanv]

Looking into https://github.com/scikit-image/scikit-image/blob/628adaa/tools/generate_requirements.py#L32-L34, it seems like you don't really want build deps in the terms of PEP 517 but runtime deps that happen to be needed too.

Both use-cases are important to us. See requirements/build.txt vs requirements/optional.txt.

[webknjaz]

@stefanv ah, I didn't realize that you maintain the entries in two places in sync.

FWIW I recommended to document that this is explicitly out of the scope in PEP 735.

I also noted that extending the build deps with the dependency group entries is something the build backends should eventually be able to implement eventually, making it a backend problem rather than frontend: https://discuss.python.org/t/pep-735-dependency-groups-in-pyproject-toml/39233/241. With that, you'd be able to keep the entries in dependency groups as the canonical location, not having to duplicate them in the build system section.

[charliermarsh]

@stefanv -- How much of this is covered by the fact that we support pip install -r pyproject.toml and pip compile pyproject.toml?

[stefanv]

@stefanv -- How much of this is covered by the fact that we support pip install -r pyproject.toml and pip compile pyproject.toml?

Hi @charliermarsh, thanks for pointing out that feature (BTW, I notice that the --help does not indicate that pyproject.toml is an acceptable file). That's useful for getting the environment ready for running a package in, but not for building the package. For that, we need to be able to install its build dependencies. Similarly, one may want to define the set of dependencies for building docs, etc.

[charliermarsh]

Sorry, yes, I forgot that this issue was about build dependencies. I came here after reading the --only-deps issue in pip which seemed like it would be partially solved by uv pip install -r pyproject.toml.