[Distutils] PEP 517 again

[xoviat]

For the record, I agree with the proposals made in Nathaniel's last two
emails.

2017-08-25 17:36 GMT-05:00 Nathaniel Smith <njs at pobox.com>:

> On Fri, Aug 25, 2017 at 2:26 PM, xoviat <xoviat at gmail.com> wrote:
> >> I'm more or less persuaded by Nathaniel's argument that the source
> >> directory shouldn't be on sys.path
> >
> > I do too. There should be an option in pyproject.toml to disable this
> > behavior though so that numpy can build itself.
>
> My original proposal was to leave the srcdir off of sys.path, and then
> have a key in pyproject.toml like:
>
> [build-system]
> backend-python-path = ["."]
>
> Frontends would then do something like:
>
> os.chdir(srcdir)
> # This line is new to handle the above proposal:
> sys.path[:0] = [abspath(p) for p in
> config["build-system"].get("backend-python-path", [])]
> backend = resolve_entrypoint(config["build-system"]["build-backend"])
>
> I don't have a strong opinion on whether we put this into PEP 517
> (it's pretty light weight and doesn't interact with any other features
> AFAICT), or make it a followup PEP, or start out by deferring this
> option to a dedicated build backend, like:
>
> [build-system]
> requires = ["override_backend_path"]
> build-backend = "override_backend_path"
>
> [tool.override_backend_path]
> python-path = ["."]
> real-backend = "my_awesome:backend"
>
> These are all pretty similar.
>
> I think the big question for debate is: should sys.path be
> configurable, or not configurable? IIUC, the main argument for putting
> the source directory on the path was that extra configuration options
> are annoying, so we don't want one of those, but we do want to support
> in-tree backends (even though we expect that most projects won't use
> this), so we had better put the srcdir on sys.path.
>
> My feeling is that in practice, though, that the "no configuration,
> srcdir always on sys.path" approach is not going to hold up. So first,
> obviously, the hack above works just fine to make it configurable even
> if we don't put it in any PEP, so.... in fact it's configurable no
> matter what. Plus, sooner or later, someone's going to say "hey
> distutils-sig, I have a build backend package that I want to be able
> to bootstrap itself, AND I want to put my package inside src/ for
> reasons [1], wouldn't it be nice if I could put src/ on sys.path
> without jumping through hoops?". Or someone will release a new version
> of their build backend that adds a new dependency, or one of their
> transitive dependencies will release a new version that adds a new
> dependency, and it collides with some already-released package that
> uses that build-backend, and the project suffering the collision gets
> annoyed at being told they need to rearrange their source tree
> (retroactively, in already released versions!). And they'll come here
> and say "hey distutils-sig, can solve this problem once and for all?".
> And we'll be like... uh, fixing this would take what, <5 lines of new
> code in pip? Kinda hard to argue with that.
>
> So... it'll be configurable, one way or another.
>
> And if it's configurable... then the question is about whether the
> default configuration: is it srcdir on sys.path ("opt-out"), or srcdir
> not on sys.path ("opt-in"). And it seems to me that in this case, all
> the standard criteria say it should be opt-in.
>
> If it's opt-in, then everyone using build backends distributed on PyPI
> -- which we expect to be the vast majority of project -- never have to
> think about it and it does the right thing, with no risk of collisions
> or anything. In fact the only people who have to think about it are
> the ones implementing in-tree backends, and if you're already like,
> writing a whole backend and configuring your pyproject.toml to invoke
> it, then asking you to add one more line of configuration is really
> not a big deal.
>
> OTOH if it's opt-out, then it becomes Yet Another Bad Packaging
> Default, where conscientious package authors will fret about the risk
> of collisions and write blog posts about how every project needs to
> make sure to opt-out of this as a Best Practice, and I am so, so tired
> of those.
>
> -n
>
> [1] https://blog.ionelmc.ro/2014/05/25/python-packaging/#the-structure
>
> --
> Nathaniel J. Smith -- https://vorpus.org
>
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