[Distutils] A possible refactor/streamlining of PEP 517

[Thomas Kluyver]

On Thu, Jul 6, 2017, at 11:51 PM, Paul Moore wrote:
> On reflection, I'm less concerned about this than I was. If you wanted
> to propose a stripped down version of PEP 517 which assumed it was the
> backend's responsibility to ensure reproducible isolated builds, I'd
> be willing to listen. But the proposal would need to include some
> pretty strong requirements on precisely what we're asking of backends
> - if build isolation is pip's problem to solve, then I'm happy for us
> (the pip devs) to take that responsibility, and agree hooks that we
> need to do so, but if we're assuming backends handle it for us, I
> think we need to document clearly what we're assuming (because
> frankly, the pip devs are the ones with the experience of the
> potential issues).

How does this sound to you:

"""
If build_directory is not None, it is a unicode string containing the
path to a directory where intermediate build artifacts may be stored.
This may be empty, or it may contain artifacts from a previous build to
be used as a cache. The backend is responsible for determining whether
any cached artifacts are outdated. When a build_directory is provided,
the backend should not create or modify any files in the source
directory (the working directory where the hook is called). If the
backend cannot reliably avoid modifying the directory it builds from, it
should copy any files it needs to build_directory and perform the build
there.

If build_directory is None, the backend may do an 'in place' build which
modifies the source directory. The semantics of this are not specified
here.

In either case, the backend may also store intermediates in other cache
locations or temporary directories, which it is responsible for
managing. The presence or absence of any caches should not make a
material difference to the final result of the build.
"""

> this is a case where flit is
> *not* a good example of a backend, as it avoids the nasty cases by
> design).

Agreed. Enscons is a better example, though, and Daniel seems confident
that out-of-tree builds are feasible.

Thomas