Implement prepare_metadata_for_build_wheel

[lithomas1]

We should implement this method, since if it is unavailable, pip will call build_wheel to find the metadata.

Since we don't support editable installs, if a project supports both meson and setuptools as a build system, the project will be built unnecessarily with meson before proceeding with an editable install with setup.py.

In order to do this, we just need to write .dist-info with only METADATA and WHEEL (no need for RECORD0.

[dnicolodi]

Where is the fall-back from a pep517-build to a setuptools-build when the pep517 backend does not support editable install documented? At a quick look I don't see it in pep517. If the installation is going to proceed with setuptools, what's the purpose of obtaining metadata from a different pep517 backend? Isn't it going to be discarded anyway?

[rgommers]

if a project supports both meson and setuptools as a build system

I wanted this for SciPy, but it is impossible by design, pip doesn't allow it. You can make duplicate sets of setup.py and meson.build files in your source tree, but you need a one line patch to switch build backends. Using pip install --no-pep-517 does not work.

[rgommers]

Since we don't support editable installs, if a project supports both meson and setuptools as a build system, the project will be built unnecessarily with meson before proceeding with an editable install with setup.py.

We should support it soon. But other than that, is this actually happening? That seems like a pip bug, since they have on purpose not even made --no-pep-517 work.

[dnicolodi]

The only way I know to make this work is to call python setup.py develop explicitly.

[rgommers]

When we released SciPy 1.9.0 the sdist had build-backend = 'mesonpy', and I told all distributors that still had trouble with building (e.g. due to less flexible BLAS support) to carry a patch to comment out that one line, and then things do work as expected with setuptools.

[dnicolodi]

@rgommers Sure. I meant that calling setup.py directly is the only way without patching pyproject.toml. Of course if you change the build backend declaration, the right backed is picked up by the frontend.

[lithomas1]

if a project supports both meson and setuptools as a build system

I wanted this for SciPy, but it is impossible by design, pip doesn't allow it. You can make duplicate sets of setup.py and meson.build files in your source tree, but you need a one line patch to switch build backends. Using pip install --no-pep-517 does not work.

pip is actually smart enough to fall back to setup.py, when it can't find the build_editable hook.

But I guess this doesn't matter much since the editable installs PR got updated. I'm happy to wait for that instead if its landing soon.

[FFY00]

Implementing this is possible, but the commitment to use the metadata from the prepare_metadata_for_build_wheel hook is annoying, and makes things a bit more complicated.

Actually, we could consider simply checking the metadata from the prepare_metadata_for_build_wheel hook against the metadata we were going to generate and error out if they differ.

[FFY00]

To be clear, this can be implemented, and would be a nice improvement, just not really high-priority right now, so I am re-opening.

The specific workflow issue mentioned is solved by editable installs, but the proposal itself is still relevant.

[rgommers]

Can you explain why it's an improvement? What does it enable that isn't possible (or slower, or ...) now?

[FFY00]

Fetching the metadata for the package. See https://pypa-build.readthedocs.io/en/stable/api.html#build.util.project_wheel_metadata, and https://github.com/jaraco/jaraco.packaging as an example use-case. This is especially relevant in our build backend because we are mainly target compiled, often big or even very big, projects, and having to compile the full project to get the metadata is not great.