Support for build-and-run-time dependencies

[jdemeyer (Jeroen Demeyer)]

@bernatgabor

What’s stopping you from specifying such dependencies under both pyproject.toml requires and install_requires?

There is no really fundamental reason why not, but there are some issues:

1. Those packages are possibly built twice which is annoying if the build takes a while (this is likely since we’re talking about C extensions here).

2. There is no way to guarantee that the same version of the package is used at build-time and run-time. This may cause ABI problems.

3. It goes against Don’t-Repeat-Yourself. Keeping track of the same dependencies in two unrelated places is not so clean.

4. The “gut feeling” that these dependencies really shouldn’t be considered as part of the build system. The wording of PEP 518 is about build system/build tools but that doesn’t apply to these dependencies.