PEP 517 interface (build_meta) should not specify setuptools in build dependencies

[uranusjr]

From pypa/pyproject-hooks#15.

Quoting @takluyver’s comment:

I think that what you hit is a bug in the setuptools build backend: I don't think it should be returning the values from build-system.requires at all. The spec says that the get_requires_for_build_* hooks should return:

an additional list of strings containing PEP 508 dependency specifications, above and beyond those specified in the pyproject.toml file

To me, that means that the backend shouldn't include the build-system.requires values, because the frontend should have already found those before calling the hook.

[gaborbernat]

Setuptools at the moment does not have a TOML parser. To resolve this though we would need to vendor one into it though. Maybe the build frontend should pass on build requires elements 🤔 but that would be PEP-517 change. @takluyver? @dstufft otherwise now all build frontends need to be able to parse TOML; and need to be able to locate the pyproject.toml.

[takluyver]

I don't think this needs setuptools to parse TOML. The frontend gets the build-system.requires from the TOML file and installs those. Then it calls the build system to ask for additional build requirements. In setuptools' case, that should be whatever is specified in setup_requires.

[takluyver]

(Though if setuptools isn't currently reading the TOML file, I must have misunderstood where this issue is coming from)

[gaborbernat]

setuptools does not read the file, so always responds with setuptools and wheel. It's hardcoded. To be able to know what additional it needs to specify it would need to do so though.

[uranusjr]

What happens if setup_requires lists setuptools itself? Does setuptools automatically drop it? What happens if the currently-installed setuptools does not match it?

[SpecLad]

Wasn't this effectively fixed by #1594? It removed setuptools from the list of build dependencies, and while it didn't remove wheel, I don't think wheel should be removed anyway, as it is a requirement for building wheels.

[jaraco]

Interesting. I was previously unaware of these layers. So it seems that any project relying on setuptools and PEP 517/518 automatically gets wheel? And users don't need to specify wheel in the build-system.requires? And they don't even have to download wheel when building an sdist? Even better!

I'm going to close this issue as fixed, but I welcome more discussion if the current implementation isn't satisfactory.

[SpecLad]

And users don't need to specify wheel in the build-system.requires?

I actually found this ticket while I was researching this very question. And my conclusion was that you do need to specify wheel if you want to be compatible with the versions of pip that supported PEP 518, but not PEP 517 (that is, >=10, <19). For pip>=19 it's no longer necessary.