[Distutils] Finishing up PEP 517

[Nick Coghlan]

On 25 June 2017 at 17:26, Nathaniel Smith <njs at pobox.com> wrote:
> On Sat, Jun 24, 2017 at 7:57 AM, Thomas Kluyver <thomas at kluyver.me.uk> wrote:
>> Nick has merged that PR, and the updated PEP is visible here:
>>
>> https://www.python.org/dev/peps/pep-0517/
>>
>> Hopefully we're nearing a consensus on this now. If you're interested,
>> please do have a read through the latest version.
>>
>
> Reading through it just now, there were some bits that gave me the
> impression that prepare_build_wheel_files must *always* be called
> before calling build_wheel (e.g. "Because the wheel will be built from
> a temporary build directory, build_wheel may create intermediate files
> in the working  directory, and does not need to take care to clean
> them up."). If we end up keeping prepare_build_wheel_files, then I
> think we should add some text making clear exactly which paths are
> legal.

Doing out-of-tree builds is completely optional for frontends, so the
possible build paths are:

In-place build:
* just call build_wheel on the source tree

Out-of-tree build (via sdist):
* call build_sdist
* unpack it into the build directory
* call build_wheel on the build directory

Out-of-tree build (prepare hook defined):
* call prepare_wheel_build_files
* call build_wheel on the prepared build directory

The critical constraint on front-ends is that they need to ensure the
appropriate dependencies are present before calling the affected hooks
- beyond that, they're entirely free to do things however they like
(e.g. always build in-place, always go via the sdist hook, or some
combination there-of).

Aside from not producing an archive, the bit that makes
`prepare_wheel_build_files` notably different from `build_sdist` is
that it relies on `get_build_wheel_requires` *not* on
`get_build_sdist_requires`:
https://www.python.org/dev/peps/pep-0517/#build-environment

Cheers,
Nick.

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