[Distutils] PEP for specifying build dependencies

[Donald Stufft]

> On May 12, 2016, at 8:31 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:
> 
> We could also keep semantics-version, and just put it inside [build-system].
> 
> Either way, by allowing access to the [tool.*] namespace without any
> other version check, the key constraint we're placing on ourselves is
> a commitment to only making backwards compatible changes at the top
> level of the schema definition, and that should be a feasible promise
> to keep. While I can't conceive of an eventuality where we'd need to
> break such a promise, even if we did, the change could be indicated by
> switching to a different filename.

I don't think we should put it inside of [build-system], largely because I
think the chances we ever need to increment the version is very small, and I
feel like putting it inside of [build-system] means we'll then need one for
any other top level key we put in. Each additional one is additional complexity
and increases the chance that some tool doesn't accurately check every single
one of them. Putting it inside of [package] made some sense, because that was
going to be a container for all of the things that one particular group of
people (distutils-sig / PyPA) "managed" or cared about but I think that putting
it on each individual sub section is just overkill.

We can easily stick it at the top level of the file and just explicitly state
that the [tool.*] namespace is exempt from deriving any sort of meaning from
the semantics-version value. I think that is easier to not screw up (only one
check, vs N checks) and I think that it looks nicer too from a human
writing/editing POV if we ever do bump the version and force people to write it
out:

    semantics-version = 2

    [build-system]
    requires = [
        "setuptools",
        "pip",
    ]

    [test-runner]  # Just an example
    command = "py.test --strict"
    requires = [
        "pytest",
    ]

    [tool.pip]
    index-url = "https://index.example.com/simple/"

But honestly, I'm of the opinion we could probably just ditch it. I don't think
it'll be hard to maintain compatibility within the keywords we pick in this
file and I worry that by including it in something that we expect humans to
write, we provide an incentive to using it when perhaps we could think up a
better, backwards compatible syntax. The main argument in favor of adding it
now with an implicit default of `1` is that if I'm wrong and we end up needing
it, including it now will mean that projects are actively checking the version
number so we can safely increase it with the desired effect. If we don't
include it now, then even if we add it at a later date nothing will be checking
to see if that changed or not.

-----------------
Donald Stufft
PGP: 0x6E3CBCE93372DCFA // 7C6B 7C5D 5E2B 6356 A926 F04F 6E3C BCE9 3372 DCFA

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