[Distutils] PEP 517 again

[Daniel Holth]

Don't be afraid of extras. They have not been PEP'd but are required for a
useful Python packaging system. install_requires is just a shorthand for
extras with the "" empty string as their name. Too bad we couldn't agree on
; vs : as the delimiter though.

On Thu, Aug 31, 2017 at 2:04 PM Nathaniel Smith <njs at pobox.com> wrote:

> On Thu, Aug 31, 2017 at 8:41 AM, Chris Barker - NOAA Federal
> <chris.barker at noaa.gov> wrote:
> > The package manager should manage the package, not built it, or change
> it.
> >
> > Surely the build system should know how to correctly name the wheel it
> builds.
>
> It's probably worth mentioning the specific problem that motivated pip
> to start doing this.
>
> It used to be standard, and is still quite common, for setup.py
> scripts to contain stuff like:
>
>     install_requires = [...]
>     if sys.version_info < (3, 4):
>         install_requires += [...]
>     if platform.python_implementation() == "PyPy":
>         install_requires += [...]
>
>     setup(..., install_requires=install_requires)
>
> This kind of logic in setup.py worked fine in the old days when all
> you did was 'setup.py install', but then wheels came along and
> retroactively turned lots of setup.py scripts from working into
> broken. The problem is that with this kind of setup.py, setuptools has
> *no idea* that the install_requires you gave it would have been
> different if you had run setup.py with a different version of Python,
> so when it has to assign Python tags to a built wheel it guesses wrong
> and uses ones that are too general.
>
> The right way to do this is to use PEP 508 environment markers:
>     https://www.python.org/dev/peps/pep-0508/#environment-markers
> or the non-standard extras hack:
>
> https://wheel.readthedocs.io/en/latest/#defining-conditional-dependencies
> Both of these let you export the whole requirements-choosing logic
> into the wheel metadata, so that it can be evaluated at install time
> instead of build time.
>
> But it will take a while for existing setup.py files transition to
> using those, and in the mean time pip can't assume that a random wheel
> generated by 'setup.py bdist_wheel' has accurate Python tags.
>
> Hopefully new legacy-free backends will get this right from the start.
> For example flit makes it impossible to get this wrong.
>
> -n
>
> --
> Nathaniel J. Smith -- https://vorpus.org
> _______________________________________________
> Distutils-SIG maillist  -  Distutils-SIG at python.org
> https://mail.python.org/mailman/listinfo/distutils-sig
>
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