On Fri, 22 Jun 2018 at 09:35 Pradyun Gedam <pradyu...@gmail.com> wrote:
> Hey everyone! > > In PEP 518, it is not clearly specified how a project that has a > pyproject.toml > file but has no build-system.requires should be treated (i.e. build-system > table). > > In pip 10, such a pyproject.toml file was allowed and built with > setuptools > and wheel, which has resulted in a lot of projects making releases that > assumed > that such a pyproject.toml file is valid and they use setuptools and wheel. > I understand that at least pytest, towncrier and Twisted might have done > so. > This happened since these projects have included configuration for some > tool in > pyproject.toml (some of which use only pyproject.toml for configuration -- > black, towncrier). > > There's a little bit of subtlety here, in pip 10's implementation: adding a > pyproject.toml file enables a new code path that does the build in > isolation > (in preparation for PEP 517; it's a good idea on it's own too) with only > the > build-system.requires packages available. When the build-system.requires > key > is missing, pip falls back to assuming it should be ["setuptools", > "wheel"]. > The in-development version of pip currently prints warnings when the key is > not specified -- along the lines of "build-system.requires is missing" + > "A future version of pip will reject pyproject.toml files that do not > comply > with PEP 518." and falls back to legacy behavior. > > Basically, pip 10 has a distinction between a missing pyproject.toml and > build-system.requires = ["setuptools", "wheel"] and the PEP doesn't. > However, > the PEP's precise wording here would help inform the debate about how pip > should behave in this edge case. > I think the precise wording is there by not having any wording. ;) If 'requires' was meant to be optional then the PEP would have said that. > > I can think of at least 2 options for behavior when build-system.requires > is > missing: > > 1. Consider a missing build-system.requires equivalent to either a missing > pyproject.toml or build-system.requires = ["setuptools", "wheel"]. > > 2. Making the build-system table mandatory in pyproject.toml. > > I personally think (2) would be fine -- "Explicit is better than implicit." > And I think that's what pip's warning is saying the future will be, but they aren't quite yet ready to be that strict yet with their users (which I can understand). > > It'll be easy to detect and error out in this case, in a way that it's > possible > to provide meaningful information to the user about what to do here. > However, > this does mean that some existing releases of projects become > not-installable, > which is concerning; I do think the benefits outweigh the costs though. > > Thoughts on this? > I think what pip has planned with the warning makes sense.
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