pip has multiple environment marker parsers

[gsnedders]

The various projects pip vendors in leads it to have three environment marker parsers:

• pip/_vendor/pkg_resources/__init__.py (line 1583/4 for the exposed API)
• pip/_vendor/packaging/markers.py
• pip/_vendor/distlib/markers.py

pip uses the pkg_resources and distlib ones, as far as I can tell. Unfortunately, packaging is the one I believe we want to be using (it's compatible with PEP 345, PEP 508, and other relevant PEPs, as well as being backwards compatible with both pkg_resources and distilb with the sole exception of distilb's undocumented platform_in_venv marker, which I can find no evidence of anyone using anywhere).

More recent releases of setuptools have pkg_resources just call packaging, which deals with that duplication (and hence we should solve that by merely upgrading our version of pkg_resources).

When it comes to distilb, we should move all our code that uses that to one of the other ones so then we're using a single implementation in all PyPA stuff, and then we have consistency and interoperability.

[xavfernandez]

Where is pip using distlib's markers ?

[gsnedders]

@xavfernandez in pip.req.req_install.InstallRequirement.match_markers

[sergray]

@dstufft @gsnedders could you please take a look at #3781? it seems to be a regression preventing installation of some published packages using 8.1.2