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Excerpts from Robert Collins's message of 2015-03-17 12:54:00 +1300:
> I've raised this in reviews 157135 and 153966, but I think it deserves
> a thread of its own.
> 
> I think universal wheels are useful - they are simple to build and
> publish - we don't need to do one wheel per Python version.
> 
> However, right now I'm fairly sure that we're not exporting the
> requirements from requirements-py2 / requirements-py3 as environment
> markers (see PEP-426).
> 
> That means that a wheel built on Python2 for a pbr project using
> requirements-pyN files, even if marked as a universal wheel, will only
> have the requirements for the Python2 deps.

You're right that they only include the requirements for python 2. We
try not to mark those packages as "universal" for that reason.

> 
> This is broken - I've filed a bug about it (http://pad.lv/1431529).
> 
> I think we should deprecate and remove the requirements-pyN files and
> instead use environment markers directly in requirements.txt. That
> will then flow into wheels and things should just work (plus we can
> delete more pbr code).
> 
> I haven't tested yet (and someone should) that it does all JUST WORK,
> but thats easy: put an environment marker in a requirements.txt file
> like so:
> 
>  argparse; python_version < '3'

I think the last time this came up the feature wasn't available in pip
yet, and so using separate files was the work-around. Are environment
markers fully supported by pip/setuptools/whatever now?

If so, an option would be to have pbr recognize the version-specific
input files as implying a particular rule, and adding that environment
marker to the dependencies list automatically until we can migrate to a
single requirements.txt (for which no rules would be implied).

Doug

> 
> Build a wheel, unpack it(unzip foo.whl) and cat
> foo/*,dist-info/METADATA to check that the marker carried through.
> 
> -Rob
>