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

Nathaniel Smith njs at pobox.com
Sat Jun 3 01:40:43 EDT 2017

On Fri, Jun 2, 2017 at 8:38 PM, Donald Stufft <donald at stufft.io> wrote:
> On Jun 2, 2017, at 10:14 PM, Nathaniel Smith <njs at pobox.com> wrote:
>> So far my belief is that packages with expensive build processes are
>> going to ignore you and implement, ship, document, and recommend the
>> direct source-tree->wheel path for developer builds. You can force the
>> make-a-wheel-from-a-directory-without-copying-and-then-install-it
>> command have a name that doesn't start with "pip", but it's still
>> going to exist and be used. Why wouldn't it? It's trivial to implement
>> and it works, and I haven't heard any alternative proposals that have
>> either of those properties. [1]
> If someone wants to implement a direct-to-wheel build tool and have it
> compete with ``pip install .`` they’re more than welcome to. Competition is
> healthy and at the very worst case it could validate either the idea that
> direct-to-wheel is important enough that people will gladly overcome the
> relatively small barrier of having to install another tool and then we have
> data to indicate maybe we need to rethink things or it could validate the
> idea that it’s not important enough and leave things as they are.
> I went and looked through all 105 pages of pip’s issues (open and closed)
> and made several searches using any keyword I could think of looking for any
> issue where someone asked for this. The only times I can find anyone asking
> for this were you and Ralf Gommers as part of the extended discussion around
> this set of PEPs and I’ve not been able to find a single other person asking
> for it or complaining about it.

That's because until now, the message that everyone has received over
and over is that the way you install a package from a directory on
disk is:

  cd directory
  python setup.py install

and this does incremental builds. (My experience is that even today,
most people are surprised to learn that 'pip install' accepts
directory paths.)

In our glorious PEP 517 future, we have to teach everyone to stop
using 'setup.py install' and instead use 'pip install .'. This switch
enables a glorious new future of non-distutils-based build systems and
fixes a bunch of other brokenness at the same time, hooray, BUT
currently switching to 'pip install' also causes a regression for
everyone who's used to incremental builds working.

Ralf and I noticed this because we were looking at getting a head
start on the glorious future by making 'pip install' mandatory for
numpy and scipy. The reason no-one else has noticed is that we're
among the few people that have tried using 'pip install' as their
standard install-from-working-tree command. But soon there will be

> However, what I was able to find was what appears to be the original reason
> pip started copying the directory to begin with,
> https://github.com/pypa/pip/issues/178 which was caused by the build system
> reusing the build directory between two different virtual environments and
> causing an invalid installation to happen. The ticket is old enough that I
> can get at specifics it because it was migrated over from bitbucket. However
> the fact that we *used* to do exactly what you want and it caused exactly
> one of problem I was worried about seems to suggest to me that pip is
> absolutely correct in keeping this behavior.

Hmm, it looks to me like that bug is saying that at the time, if you
ran 'python setup.py install' *inside the pip source tree*, and then
tried to run pip's test suite (possibly via 'setup.py test'), then it
broke. I don't think this is related to the behavior of 'pip install
.', and I feel like we would know if it were currently true that
running 'setup.py install' twice in the same directory produced broken
shebang lines. (Again, most people who install from source directories
are currently using setup.py install!)

The source tree copying was originally added in:


(which is dated ~2 months before that bug you found, and if I'm
reading it right tweaks a code path that previously only worked for
'pip install foo.zip' so it also works for 'pip install foo/'). AFAICT
the reason it was written this way is that pip started out with the
assumption that it was always going to be downloading and unpacking
archives, so the logic went:

1) make a temporary directory
2) unpack the sdist into this temporary directory
3) build from this temporary directory

Then, when it came time to add support for building from directories,
the structure of the logic meant that by the time pip got to step (2)
and realized that it already had a source directory, it was too late
-- it was already committed to using the selected temporary directory.
So instead of refactoring all this code, they made the minimal change
of implementing the "unpack this sdist into this directory" operation
for source directories by using shutil.copytree.

I think this chain of reasoning will feel very familiar to anyone
working with the modern pip source 5 years later...

It's absolutely true that there are cases where incremental builds can
screw things up, especially when using distutils/setuptools. But I
don't think this is why pip does things this way originally :-).

> It’s not that I don’t trust the backend, it’s that I believe in putting in
> systems that make it harder to do the wrong thing than the right thing. As
> it is now building in place correctly requires the build backend to do extra
> work to ensure that some file that wouldn’t be included in the sdist doesn’t
> influence the build in some way. Given that I’m pretty sure literally every
> build tool in existence for Python currently fails this test, I think that
> is a pretty reasonable statement to say that it might continue to be a
> problem into the future.
> Copying the files makes that harder to do (but still easier than always
> going through the sdist). If you want to argue that we should always go
> through the sdist and we shouldn’t have a copy_files hook, I’m ok with that.
> I’m only partially in favor of it as a performance trade off because I think
> it passes a high enough bar that it’s unlikely enough for mistakes to be
> made (and when they do, they’ll be more obvious).

What do you think of letting build backends opt-in to in-place builds?

>> Other unresolved issues:
>> - Donald had some concerns about get_wheel_metadata and they've led to
>> several suggestions, none of which has made everyone go "oh yeah
>> obviously that's the solution". To me this suggests we should go ahead
>> and drop it from PEP 517 and add it back later if/when the need is
>> more obvious. It's optional anyway, so adding it later doesn't hurt
>> anything.
> My main concern is the metadata diverging between the get_wheel_metadata and
> the building wheel phase. The current PEP solves that in a reasonable enough
> way (and in a way I can assert against). My other concerns are mostly just
> little API niggles to make it harder to mess up.
> I think this one is important to support because we do not to be able to get
> at the dependencies, and invoking the entire build chain to do that seems
> like it will be extraordinarily slow.

It's only slow in the case where (a) there's no wheel (obviously), and
(b) after getting the dependencies we decide we don't want to install
this sdist after all. I imagine numpy for example won't bother
implementing get_wheel_metadata because we provide wheels for all the
platforms we support and because we have no dependencies, so it is
doubly useless AFAICT. But yeah in other cases it could matter. I'm
not opposed to including it in general, just thought this might be a
way to help get the minimal PEP 517 out the door.

>> - It sounds like there's some real question about how exactly a build
>> frontend should handle the output from build_wheel; in particular, the
>> PEP should say what happens if there are multiple files deposited into
>> the output dir. My original idea when writing the PEP was that the
>> build frontend would know the name/version of the wheel it was looking
>> for, and so it would ignore any other files found in the output dir,
>> which would be forward compatible with a future PEP allowing
>> build_wheel to drop multiple wheels into the output dir (i.e., old
>> pip's would just ignore them). It's clear from the discussion that
>> this isn't how others were imagining it. Which is fine, I don't think
>> this is a huge problem, but we should nail it down so we're not
>> surprised later.
> How do you determine the name/version for ``pip install .`` except by
> running get_wheel_metadata or build_wheel or build_sdist?

Well, I was imagining that the semantics of 'pip install .' in a
multi-wheel world would be to install all the generated wheels :-).
But yeah, it's not really well-specified as currently written.

Possibly the simplest solution is to say that build_wheel has to
return a string which names the wheel, and then in the future we could
add build_wheel2 which is identical but returns a list of strings, and
backwards compatibility would be:

def build_wheel2(...):
    return build_wheel(...)[0]

> -n
>> [1] Donald's suggestion of silently caching intermediate files in some
>> global cache dir is unreasonably difficult to implement in a
>> user-friendly way – cache management is Hard, and I frankly I still
>> don't think users will accept individual package's build systems
>> leaving hundreds of megabytes of random gunk inside hidden
>> directories. We could debate the details here, but basically, if this
>> were a great idea to do by default, then surely one of
>> cmake/autoconf/... would already do it? Also, my understanding is the
>> main reason pip wants to copy files in the first place is to avoid
>> accidental pollution between different builds using the same local
>> tree; but if a build system implements a global cache like this then
>> surprise, now you can get pollution between arbitrary builds using
>> different trees, or between builds that don't even use a local tree at
>> all (e.g. running 'pip install numpy==1.12.0' can potentially cause a
>> later run of 'pip install numpy==1.12.1' to be corrupted). And, it
>> assumes that all build systems can easily support out-of-tree
>> incremental builds, which is often true but not guaranteed when your
>> wheel build has to wrap some random third party C library's build
>> system.
> Make it opt-in

If it's opt-in, then I might as well tell people to run 'pip
devinstall .' or 'in-place-install .' or whatever instead, and it'll
be much easier all around. But instead of making it opt-in, I'd much
rather it Just Work. It's frustrating that at the same time we're
moving to the glorious simplified future, we're also picking up a new
piece of arcane wisdom that devs will need to be taught, and another
place where numerical Python devs will roll their eyes at how the
standard Python tooling doesn't care about them. (And I totally
understand that the motivation on your end is also to make things Just
Work, but I feel like in the specific case where someone is
*repeatedly* building out of the *same source directory* – which is
the one in dispute here – we should optimize for developer

> and build a hash of the directory into the cache key so
> different file contents mean different cache objects then. I’m not really
> sold on the idea that the fact some developers haven’t decided to do it then
> it is a bad idea. Perhaps those build systems are operating under different
> constraints than we are (I’m almost certainly sure this is the case).

I think the way the constraints differ is just that they don't have
this imposed constraint that they *must* build out of an ephemeral
tempdir. If we're operating under that constraint, then your idea is
certainly worth considering. My point is that it's much easier to
remove that constraint (by switching to using a non-pip tool) than it
is to work around it, so that's my prediction for what will happen.


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

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