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Hello everyone,

This has come up a few times on IRC so we are probably well overdue for a email about it. Long story short, if your tox.ini config sets basepython [0] to `python3` and you also use py35, py36, py37, or py38 test targets there is a good chance you are not testing what you intend to be testing. You also need to set ignore_basepython_conflict to true [1].

The reason for this is basepython acts as an override for the python executable to be used when creating tox virtualenvs. `python3` on most platforms indicates a specific python3 version: Ubuntu Xenial 3.5, Ubuntu Bionic and CentOS 8 3.6, and so on. This means that even though you are asking for python 3.7 via py37 you'll get whatever version `python3` is on the running platform. To address this we can set ignore_basepython_conflict and tox will use the version specified as part of the target and not the basepython override.

You might wonder why using basepython is useful at all given this situation. The reason for it is the default python used by tox for virtualenvs is the version of python tox was installed under. This means that if tox is running under python2 it will use python2 for virtualenvs when no other version is set. Since many software projects are now trying to drop python2 support they want to explicitly force python3 in the default case. basepython gets us halfway there, ignore_basepython_conflict the rest of the way.

[0] https://tox.readthedocs.io/en/latest/config.html#conf-basepython
[1] https://tox.readthedocs.io/en/latest/config.html#conf-ignore_basepython_conflict

Hopefully this helps explain some of tox's odd behavior in a beneficial way. Now go and check your tox.ini files :)

Clark