[Distutils] Fwd: Re: PEP 517 again

[Nick Coghlan]

On 25 August 2017 at 00:51, Paul Moore <p.f.moore at gmail.com> wrote:
> 4. The point of all this is that the definition of build_directory
> says "If build_directory is None, the backend may do an 'in place'
> build which modifies the source directory and may produce different
> results from those that would be obtained by exporting an sdist
> first". That's the bit that bothers me a lot - if we lose
> build_directory, we lose the option for pip to request that a backend
> produce a wheel "equivalent to producing an sdist first". And that
> capability is something I think pip should require[2]. I don't
> particularly care who specifies the actual directory (as per your
> comment, I thought someone else wanted this, for caching or something)
> but I *do* care about telling backends we don't want "different
> results from those that would be obtained by exporting an sdist
> first".

That's a phrasing error in the PEP - ideally, backends should always
produce consistent results regardless of whether or not the frontend
builds an sdist first. What that phrasing was attempting to
acknowledge is that actually ensuring sdist/wheel consistency may have
some extra requirements (like the git CLI) that may not apply to the
"just give me a usable wheel, I don't care whether it's publication
quality or not" case.

The key thing that changed my mind on whether or not we needed to
include the build_directory parameter from the start was actually the
NotImplementedError discussion: as soon as we permitted backends to
raise NotImplementedError for any non-None build_directory setting,
we'd effectively made supporting the feature optional.

And if supporting the feature's going to be optional anyway, then the
*whole point* of defining the backend API in Python rather than as a
CLI was to make it easy for us to add new optional methods later,
which means we can start without build_directory support, and if we
find we miss it for some specific reason, then we can add it back as a
suitably named optional method like "build_wheel_variant" (if the
use-case is cross-compiling for multiple ABIs without clobbering
intermediate artifacts), or "build_wheel_with_artifact_cache" (if the
use case is to allow the frontend to ask that the backend cache
intermediate values in a particular place).

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia