[Distutils] Provisionally accepting PEP 517's declarative build system interface

Nathaniel Smith njs at pobox.com
Mon May 29 15:09:48 EDT 2017

On Mon, May 29, 2017 at 7:26 AM, Donald Stufft <donald at stufft.io> wrote:
> On May 29, 2017, at 3:05 AM, Nathaniel Smith <njs at pobox.com> wrote:
> > I think there's some pip bug somewhere discussing this, where Ralf
> > Gommers and I point out that this is a complete showstopper for
> > projects with complex and expensive builds (like scipy). If 'pip
> > install .' is going to replace 'setup.py install', then it needs to
> > support incremental builds, and the way
> > setup.py-and-almost-every-other-build-tool do this currently is by
> > reusing the working directory across builds.
> Wouldn’t supporting incremental builds the way ccache does work just fine?
> Have a per build tool cache directory somewhere that stores cached build
> output for each individual file keyed off a hash or something? (For that
> matter, if someone wants incremental rebuilds, couldn’t they just *use*
> ccache as their CC?).

With a random numpy checkout on my laptop and a fully-primed ccache,
some wall-clock timings:

no-op incremental build (python setup.py build): 1.186 seconds

python setup.py sdist: 3.213 seconds
unpack resulting tarball: 0.136 seconds
python setup.py build in unpacked tree: 7.696 seconds

So ccache makes the sdist-and-build a mere 10x slower than an in-place
incremental build.

ccache is great, but it isn't magic. It can't make copying files
faster (notice we're already 3x slower before we even start
building!), it doesn't speed up linking, and you still need to spawn
all those processes and hash all that source code instead of just
making some stat() calls.

Also, this is on Linux. The numbers would look much worse on Windows,
given that it generally has much higher overhead for unpacking
tarballs and spawning lots of processes, and also given that ccache
doesn't support MSVC!

Also also, notice elsewhere in the thread where Thomas notes that flit
can't build an sdist from an unpacked sdist. It seems like 'pip
install unpacked-sdist/' is an important use case to support...


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

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