ENH: add pyproject.toml - #14053

Hmmm, from Pauli's mail:

[...] there is some end-user impact (would e.g. have to be mentioned in release notes).

You probably also need to disable build isolation in numpy-wheels, to
manage the dependency versions manually. It probably also should be
mentioned in install instructions somewhere.

The main thing to watch out with introducing pyproject.toml is that
build-time dependencies for non-pure-Python packages work properly only
with pip 19 (unless they have wheels), which is fairly
recent.

OK, since nobody spoke up against it. I will put this in to try it out, if it creates downstream issues we can revert it. Towncrier does not seem to be responsive about adding --pyproject option or so, but I expect it will happen at some point, so that by the time we actually make a release it does not matter much if this is in or not.

Oooops, had forgotten about my own comment. Will make an issue to document this ://

In https://issues.apache.org/jira/browse/ARROW-8983 we observed that on Py3:

python -m pip download --dest /tmp numpy==1.17.5 --no-binary :all:

finishes immediately, while:

python -m pip download --dest /tmp numpy==1.18.0 --no-binary :all:

takes a while to complete. Could this PR be causing this? If yes: is this something that should be fixed or it's a known behavior and is considered WAI?

Probably related, though not sure if that is expected or not, maybe ask at pip? You can get the old behaviour by using:

python3 -m pip download --dest /tmp numpy==1.18.0 --no-binary :all: --no-use-pep517

We don't technically need the pyproject.toml, but it seems like the right thing to have nowadays?

Thank you. Asked in pypa/pip#8387.

Strictly speaking, Numpy does need pyproject.toml if you want to build without having first installed Cython manually, as pre-generated sources are no longer included in the sdist. That pip download apparently needs to run setup.py is then a bit annoying, as it then has to also pull the build deps.