[Distutils] PEP 517 again

[Nathaniel Smith]

On Sat, Aug 26, 2017 at 6:30 PM, C Anthony Risinger
<c at anthonyrisinger.com> wrote:
> On Aug 26, 2017 5:13 PM, "Nathaniel Smith" <njs at pobox.com> wrote:
>
> On Sat, Aug 26, 2017 at 1:47 PM, C Anthony Risinger
> <c at anthonyrisinger.com> wrote:
>
> Sure sure, I understand all that, and why we think we need some special
> error signal from `build_sdist`, as currently written.
>
> What I'm suggesting, is maybe calling `build_sdist` without knowing if it
> can succeed is already a mistake.
>
> Consider instead, if we make the following small changes:
>
> 1. `get_requires_for_build_*` is passed the sdist and wheel directories,
> just like `build_*`, giving them the chance to actually look at tree before
> deciding what other reqs might be necessary.

That's not a change, that's how it works :-).

> 2. `get_requires_for_build_*` returns None to signal `build_*` is
> unsupported (superceded by static reqs defined in TOML) and [...] to signal
> support (can be empty).
>
> 3. `get_requires_for_build_*` assumed to return None if missing (so optional
> and implies no support).

This is what I originally proposed, except you use None where I use
NotImplemented, which has the disadvantages I noted earlier. Also,
people didn't like the missing get_requires_for_build_* being treated
as no-support, which makes sense, since we expect that
get_requires_for_build_* won't be used very often. But one can switch
the default here without affecting much else. The reason we want to
let build_sdist report failure is just for convenience of backends who
don't have any other reason to implement get_requires_for_build_sdist.

> 4. sdist reqs = `get_requires_for_build_sdist` (dynamic) + ??? (static)
>
> 5. wheel reqs = `get_requires_for_build_wheel` (dynamic) +
> `build-system.requires` (static)

build-system.requires contains the requirements that are always
installed before we even try importing the backend, so they're
available to all backend hooks equally.

> 6. If no reqs are found for sdist (no declared reqs in TOML and
> `get_requires_for_build_sdist` is missing or returns None), then
> `build_sdist` is unsupported.
>
> 7. If no reqs are found for wheel (no declared reqs in TOML and
> `get_requires_for_build_wheel` is missing or returns None), then
> `build_wheel` is unsupported. This one is a spec violation because at least
> one req is expected here (the backed itself).

The TOML requires aren't really useful as a signal about whether sdist
specifically is supported. Plus I think we probably want to leave
no-requires-in-TOML as a valid option for saying "I don't need
anything installed" (maybe because the backend is shipped inside the
source tree) rather than overloading it to have extra meanings.

-n

-- 
Nathaniel J. Smith -- https://vorpus.org