The problem is that sys.implementation is relatively new, so there's no standard way to retrieve the implementation version prior to that addition to the language spec. I agree the PEP should be explicit that that tag needs to reflect the implementation's *ABI* stability, which may or may not align with the Python language version.

We're going to need an update to the compatibility tagging spec to sort out the Linux distro compatibility issues anyway, so perhaps someone is prepared to tackle a revision to the compatibility tagging PEP?

(The relevant repo for draft specs is https://bitbucket.org/pypa/pypi-metadata-formats - although it occurs to me that could use some tags to reflect published versions of accepted PEPs)

Cheers,
Nick.