"Current Python" factor

[adamchainz]

Using GitHub Actions, my projects use one CI run per Python version. Within these runs I'd want to run just the environments matching the current Python version.

I currently do this with a bit of shell:

ENV_PREFIX=$(tr -C -d "0-9" <<< "${{ matrix.python-version }}")
TOXENV=$(tox --listenvs | grep "^py$ENV_PREFIX" | tr '\n' ',') python -m tox

In this discussion on django/daphne, I was introduced to tox-factor. It seems it would be possible to shorten my shell down to:

python -m tox -f py$(python --version 2>&1 | cut -c 8,10)

I'd like to suggest supporting this syntax to declare "all environments that match the current version of Python":

tox -f py

This would be analogous to tox -e py, which means "create a tox environment whose specifier is the current version of python". (This is only the current version, e.g. py38 even when it doesn't exist in defined environments and only as e.g. py38-djangoXX...).