Call for clarification: acceptable values for build-system.requires in pyproject.toml

Environment

• pip version: 19.0.3
• Python version: 3.7
• OS: GNU/Linux

Description

It’s not clear nor is clearly specified by PEP518. But I was having a need to have an in-tree build back-end. So I’ve hacked it via installing with a relative path. And it works! Yet, @pradyunsg has pointed out that the way I used probably doesn’t conform to PEP 508. So I tried some other ways to point to the in-tree distribution. And those didn’t work.

How to Reproduce

(this works)

[build-system]
requires = ["./build-aiohttp", ]
build-backend = "build_aiohttp.api"

# not yet supported, so it doesn't influence anything, it's for forward-compat:
backend-path = "./build-aiohttp"

But if instead of "./build-aiohttp" in requires I try any of "file://build-aiohttp", "file:///./build-aiohttp", "build_aiohttp @ file://./build-aiohttp", "build_aiohttp @ file:./build-aiohttp" pip fails to recognize those as installables.

Expected behavior

I don’t know what to expect. The method which works seems to be undefined in PEPs so I probably shouldn’t rely on it.

Pip may either decide to improve the filtering of requires option or document it being permissive…

P.S. Oh and, by the way, I was able to test my other PEP517 backend outside of the project tree via

[build-system]
requires = ["../fortunate_pkg"]

so this relative path feature proves to be quite useful for development/debugging purposes.

[chrahunt]

Agreed, especially since this use case is covered by intreehooks and (soon) backend-path.

[pradyunsg]

I’m inclined to say let’s actually abort in this edge case.

I don’t want people trying things they shouldn’t and it sort-of working.