[Distutils] Wheel "build tag"

[Bohuslav Kabrda]

Hi all,
the wheel package format, as defined in pep 427 specifies, that wheel name can contain an optional "build tag" [1]. This build tag is supposed to be a tie-breaker if two wheels have the same version. I decided that I would try to send a patch to upstream wheel project [2] to support adding this tag.
I have a few questions and maybe someone here (Nick?) can help me with this:
- Where is the "build tag" supposed to be stored in a built distribution? The pep doesn't seem to specify this. Should it be somewhere in METADATA or pydist.json?
- How should the bdist_wheel command be told to use a "build tag"? By using some commandline option, e.g. "--build-tag 3"?

My motivation for doing this is:
For Fedora's Python 3.4 packaging, I'll have to unbundle the bundled setuptools and pip wheels and make ensurepip work differently. The best approach seems to be to make python3 require setuptools and pip (in other words, they'll always install along), so if user runs "python3 -m ensurepip", ensurepip won't need to do anything. This gets tricky if the same command is run in a virtualenv, since I don't have the bundled wheels. I've discussed this with Nick and we've agreed that the best solution would be to recreate the wheels from the system installed RPMs dynamically and then installing these wheels into virtualenv (I've already done some progress on this front with a small project that I call rewheel [3]). I'm now working on a downstream patch of ensurepip, that would use rewheel to do this.
Now let me explain a bit about how we package setuptools/pip in Fedora. When there is a security issue or just a bug in a package, we don't upgrade it to latest upstream version in stable Fedora releases, since as a distribution we try to maintain API/ABI stability. We rather bump the "release" (which is sort of downstream version of the package). E.g. if there is a bug in package pip-%{version}-%{release}, we fix it and bump it to pip-%{version}-%{release+1}. But that means that if we fix a package and someone runs "python -m ensurepip --upgrade", he won't get the fix into virtualenv because the version is the same. That's why I'd like to use Fedora's release as the "build tag", so that this use case would work. I hope this all makes some sense :)


Thanks!

-- 
Regards,
Bohuslav "Slavek" Kabrda.

[1] http://www.python.org/dev/peps/pep-0427/#file-name-convention
[2] https://bitbucket.org/dholth/wheel/
[3] https://github.com/bkabrda/rewheel