[Distutils] Limit package installation to a specific Python version

[Matthias Bussonnier]

Hi all,

On Fri, Jan 20, 2017 at 2:56 PM, Nathaniel Smith <njs at pobox.com> wrote:
> On Fri, Jan 20, 2017 at 1:56 PM, Lele Gaifax <lele at metapensiero.it> wrote:
>> Hi all,
>>
>> do installers like pip and conda consult trove classifiers, or more generally
>> is there a way to "mark" a package published on PyPI as installable only in a
>> Python 3 environment?
>>

>  For sdists, historically there was no way to do this, but as of a
> few months ago this was fixed and pip now *does* honor
> python_requires, and PyPI has been updated to expose this information
> so pip can see it before downloading the sdists:
>
>     https://github.com/pypa/pip/pull/3846
>     https://github.com/python/peps/pull/56
>     https://github.com/pypa/pip/pull/3877
>
> Since this is a pretty recent change, you probably want to put a
> runtime check inside your setup.py for old Python versions, so if
> someone uses an old version of pip then they will at least get a nice
> explanation.
>
> IPython's an example of a python-3-only project that uses this
> belt-and-suspenders approach:
>
> https://github.com/ipython/ipython/blob/f060158665189fd456e54a5a864080efe28b4fdf/setup.py#L29
> https://github.com/ipython/ipython/blob/f060158665189fd456e54a5a864080efe28b4fdf/setup.py#L248
>

I'm also in the (slow) process of writing the various step you can
take when python_requires is not honored by older systems and how to
mitigate that:

https://github.com/python3statement/python3statement.github.io/pull/55

Feedback (and help) would be welcome.

In particular only system where pip 9.0+ is installed will understand
the metadata now exposed by PyPI,
so when/if you fail in your setup.py, a useful tip is to ask user to
upgrade pip.

There should be some snipets of code you copy and past in your
setup.py in this PR as well.

Thanks.
-- 
M