Add a UI to set config settings for PEP 517 backends

[pfmoore]

Still to do:

• Documentation
• Tests

But it seems to basically work, so reviews are welcome 🙂

[abravalheri]

Thank you very much for having a look on this @pfmoore ❤️

[pfmoore]

Docs added. And on reflection I'm not entirely sure if this needs further tests - I think what I've added probably covers the main cases. Suggestions for extra tests would be welcome, though.

So I'm going to move this out of "Draft" mode. Further reviews would be appreciated before I merge this.

[pradyunsg]

Haven't looked at this throughly, but a few quick comments based on a quick skim. This looks pretty great overall! :)

A minor nit: call it unit/test_pyproject_config_settings.py, instead of PEP 517?

[pradyunsg]

Should we have a test that it gets called on editable hooks?

[pfmoore]

Probably... It would likely be just a small variant on this one, so I'll add it.

[pfmoore]

Good call, this picked up an error in my implementation (which took me all morning to find 🙁)! Fix (and test) incoming.

New feature suggestion: Have pip error at startup with the message "The InstallRequirement class is an abomination and I refuse to work with it" 🙂

[pfmoore]

I think I've addressed the various review comments, so unless anyone has any additional points they want to raise, I'll merge this in a day or two. Please comment if you want me to wait (even if it's just "I need some time to think about this, please give me a while longer").

[pfmoore]

OK, going to merge this now so @pradyunsg isn't waiting around for me 🙂

[mgorny]

I'm sorry for being late to the party but we'd also need a way to pass a list as the value. The only use of config_settings I know of is --global-option in setuptools, and passing more than one option requires using a list rather than str.

[pfmoore]

From PEP 517, "Build frontends SHOULD provide some mechanism for users to specify arbitrary string-key/string-value pairs to be placed in this dictionary." You could parse the string value passed for global_option in setuptools (split on whitespace or commas, parse arbitrary JSON). I considered having a way to accepting arbitrary JSON for config_settings, but decided it was over-complex for the user (correctly quoting arbitrary JSON in the shell is messy, and depends heavily on the shell...) as well as going beyond what PEP 517 suggested.

Alternatively, a PR to add an option for this would be considered. The hard part (IMO) is designing a reasonable UI.

As a final note, pip's legacy --global-option flag is never passed to setuptools in the case of pyproject-based builds, so I can guarantee you that no pip user is currently using the ability to pass a list of values via config_settings 🙂