Testing multiple Python versions with nox and pyenv

Nox is an incredible tool, I use it for all my Python projects, highly recommend it. One of the best features of nox and tools like it is parameterization. Usually this is done for the test suite portion where you want to run your test suite on many different Python versions (and maybe other things like dependency versions).

The below code goes into a file called noxfile.py typically in the root of your project directory:

import nox

# An example nox task definition that runs on many supported Python versions:
@nox.session(
    python=["3.8", "3.9", "3.10", "3.11", "3.12", "pypy3"]
)
def test(session):
    session.install(".")
    session.install("-rdev-requirements.txt")

    session.run("pytest", "tests/")

This is awesome! Now you can run nox -s test to run all variations at once or nox -s test-3.11 to target a specific Python version. The only hangup for me was when all of your Python versions were installed through pyenv. Pyenv is another incredible tool that I highly recommend if you find yourself needing to install and manage many Python versions.

Pyenv works by being higher priority in your PATH than your system Python, thus letting it inject a python stub (and python3, pip, etc) into your shells:

$ whereis python
python: /home/sethmlarson/.pyenv/shims/python

For some reason I thought pyenv was only capable of having a single version stub available at one time and thus was frustrated by having to cycle through the different Python versions. But that's totally not the case, you can specify multiple Python versions into pyenv global and have them all available through their python3.{minor} aliases which nox can discover:

# Enable all the Python versions everywhere!
$ pyenv global 3.8 3.9 3.10 3.11 3.12-dev

# Tell nox to run all test tasks available
$ nox -s test
nox > Running session test-3.8
nox > Creating virtual environment (virtualenv) using python3.8 in .nox/test-3-8
... (test output)
nox > Ran multiple sessions:
nox > * test-3.8: success
nox > * test-3.9: success
nox > * test-3.10: success
nox > * test-3.11: success
nox > * test-3.12: success

🥳 All tests ran against all Python versions! Thanks pyenv and nox 💜

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