Don't install setup_requires when run as a PEP-517 backend.

[takluyver]

Summary of changes

Under PEP 517, installing build dependencies is up to the frontend. setup_requires are already reported as build dependencies, but setuptools should not attempt to install them itself when used as a PEP 517 backend.

Closes #2303

Pull Request Checklist

• Changes have tests
• News fragment added in changelog.d. See documentation for details

[pganssle]

Why this change?

[takluyver]

The test was failing locally without it. All the other samples in this file with py_modules don't have the .py extension, and the packaging guide says it doesn't belong there.

[pganssle]

Hm.. It's disturbing that the tests are currently passing if they fail for you locally without that change, no?

[takluyver]

I think they may have been passing on master and only failed when I made the change I was interested in. As this seemed like a simple mistake and nothing else was failing, I confess I didn't try to understand what was happening.

[jaraco]

I'm inclined to accept this change, especially since it brings the codebase one step closer to fully deprecating setup_requires.

You've flagged the change not as breaking. Are we confident there aren't any use cases that will break with this change? In other words, are there users that rely on setup_requires in a PEP 517 environment?

[takluyver]

The build dependencies are reported to the frontend which is responsible for installing them in a build environment. You can disable that, e.g. with the --no-build-isolation flag for pip, in which case the caller is responsible for ensuring the build dependencies are installed in the environment where pip is running.

So I don't believe that this breaks any scenario which is meant to work. But as with any change, someone could have relied on something that wasn't meant to work. For instance:

• If someone uses pip install --no-build-isolation without all build dependencies installed, and is used to setuptools attempting to install build dependencies anyway.
• If there are packages which can be installed by easy_install but not by pip, they may not work with PEP 517.
• If setup scripts sniff what command is being run and specify setup_requires for some commands but not others, the dependencies may not be reported to the frontend.

Would you prefer that I called that breaking?

[jaraco]

Based on that description, I'm inclined to say it's not a breaking change. Thanks for adding that detail so those that are affected can understand better the context.

[jaraco]

Released as 49.5.

[takluyver]

Thanks Jason!