Are you sure setup_requires works? njsmith was somewhat dubious about it in #7867 (comment)

PEP 518 might be the solution one day, but preferably I would rather see other scientific python projects adopting it first and working around possible issues :)

I looked around a bit more and it seems that build-time dependencies are not an easy thing to solve.

I don't think that we should add cython to install_requires since we don't require it at runtime. Adding it to setup_requires will require some workaround, e.g. https://github.com/Blosc/bcolz/blob/master/setup.py#L34-L78

As you mentioned, PEP518 might be the solution. Actually scipy did something in this direction by adding a pyproject.toml and add the build-system dependencies (since version 1.0.0). This might good to go toward the same direction.

It seems like numpy and scipy both have a pyproject.toml file. This looks like it enables one to run:

pip install git+https://github.com/numpy/numpy

on a fresh virtualenv (without cython) and it installs correctly. (pip 19.3.1)

The minimum supported version for PEP518 is pip v10 I think (or maybe even later). I wonder what happens with an older pip...

I think the consensus is to have this in for 0.22.

Are we going to be okay with bumping this up to Cython 1.0 3.0 for the 0.23? Cython 1.0 3.0 will have cython/cython#3118 which we can take advantage of.

Edit Cython 3.0

Are we going to be okay with bumping this up to Cython 1.0 for the 0.23?

Isn't Cython going to be 3.0 in its next release? Anyhow, I guess our consensus was that eventhough we're going to have a cython dependency documented there with its version, but we're going to be very liberal with bumping up that dependency.

Can we move forward with the pyproject.toml here like in https://github.com/numpy/numpy/pull/14453/files?

I'm very happy to see this go forward, but I remember one reason I didn't bring it up a while back, was this: https://discuss.python.org/t/pip-19-1-and-installing-in-editable-mode-with-pyproject-toml/1553/64

I'm very happy to see this go forward, but I remember one reason I didn't bring it up a while back, was this: discuss.python.org/t/pip-19-1-and-installing-in-editable-mode-with-pyproject-toml/1553/64

Yes, that was unfortunate but it was resolved in 19.1.1 I think (and introduced in 19.1.0) so it shouldn't be too much of an issue given that only affected party is scikit-learn contributors.

Closing in favor of #15533.