using nox and uv only to test multiple python versions

[bartdorlandt]

Hi,
I'm struggling to use a pure uv only environment that can use nox for multiple python versions...

Dockerfile

FROM ubuntu:24.04

ARG DEBIAN_FRONTEND=noninteractive
ENV TZ=Etc/GMT
ENV PATH="$PATH:/root/.local/bin"
ENV NOX_DEFAULT_VENV_BACKEND=uv
ARG VERSIONS="3.12 3.11 3.10 3.9"
COPY --from=ghcr.io/astral-sh/uv:latest /uv /bin/uv

RUN uv python install ${VERSIONS} && \
    uv tool install nox[uv]

RUN apt-get update -qy && apt-get upgrade -qy && \
    apt-get install -qy --no-install-recommends \
        ca-certificates curl gnupg2 git openssh-client

noxfile.py

"""noxfile."""
from nox import session

py_versions = ["3.12", "3.11"]

@session(python=py_versions)
def tests(session):
    session.install(".")
    session.install("pytest", "pytest-cov")
    session.run("pytest")

the uv python install 3.x doesn't create any shims (like you would have with pyenv), so they are not known to the shell with python3.11 ...
I can call nox from uv per python versions, which works, but kind of beats the point of nox.

uv run --python 3.11 nox -s tests
uv run --python 3.12 nox -s tests

For each of the command the other environment would be skipped. Example:

nox > Running session tests-3.11
nox > Missing interpreters will error by default on CI systems.
nox > Session tests-3.11 skipped: Python interpreter 3.11 not found.

(therefore not specifying the different python versions in the noxfile.)
Has anyone else played with this already? (and got it to work?)
My goal is to use versions installed by uv only and preferably have it run automatically against all desired python versions.

uv version: 0.3.3

[bartdorlandt]

Additional info.

Using a simpler noxfile:

"""noxfile."""
from nox import session

@session()
def tests(session):
    session.install(".",)
    session.install("pytest", "pytest-cov")
    session.run("pytest")

it will run fine, for the specifically requested python version.

root@69e959d9538c:/src# nox -s tests
nox > Running session tests
nox > Creating virtual environment (uv) using python in .nox/tests
nox > /root/.local/share/uv/tools/nox/bin/uv pip install .
nox > /root/.local/share/uv/tools/nox/bin/uv pip install pytest pytest-cov
nox > pytest
=================================================== test session starts ====================================================
platform linux -- Python 3.12.5, pytest-8.3.2, pluggy-1.5.0
rootdir: /src
configfile: pyproject.toml
plugins: cov-5.0.0
...

Using it specifying another version is working:

root@69e959d9538c:/src# uv run --python 3.11 nox -s tests
nox > Running session tests
nox > Creating virtual environment (uv) using python in .nox/tests
nox > uv pip install .
nox > uv pip install pytest pytest-cov
nox > pytest
=================================================== test session starts ====================================================
platform linux -- Python 3.11.9, pytest-8.3.2, pluggy-1.5.0
rootdir: /src
configfile: pyproject.toml
plugins: cov-5.0.0

[charliermarsh]

Need to find a minute to play around with this before giving a confident answer. In the meantime would love to hear from others.

[samypr100]

@henryiii might have ideas on the ideal way to do this using Nox with uv as he was the author of the integration of nox with uv

[dsully]

I have the same need to run against multiple versions of Python, but ones that are built "in-house".

Currently we're using tox to drive this as opposed to another tool wrapping tox:

[tox]
envlist = prepare,test3{10,11,12},ci
ignore_basepython_conflict = true

[testenv]
base_python = python3.10
...

[testenv:test310]
depends = prepare
commands =
    pytest test --cov src
    mypy --config-file tox.ini src
    ...

[testenv:test311]
depends = prepare
commands =
    pytest test --cov src

[testenv:test312]
depends = prepare
commands =
    pytest test --cov src

...

[wpk-nist-gov]

I ran into the same issue. In the meantime, I'm just adding the paths to uv pythons to the os.environ using the following:

def add_uv_to_environ() -> None:
    import os
    import subprocess
    from pathlib import Path

    path_to_uv_pythons = (
        subprocess.check_output(
            ["uv", "python", "dir"], env={"NO_COLOR": "1", **os.environ}
        )
        .decode()
        .strip()
    )
    paths_new = ":".join(map(str, Path(path_to_uv_pythons).glob("*/bin")))
    paths_old = os.environ["PATH"]
    os.environ["PATH"] = f"{paths_new}:{paths_old}"


add_uv_to_environ()


@nox.session
def test(session): 
...

Or as a bash script shim-uv

#!/usr/bin/env bash
p=$PATH
for k in "$(uv python dir)"/*/bin; do
    p="${k}:${p}"
done

PATH=$p "$@"

and run

shim-uv nox/tox ....

[Levelleor]

@wpk-nist-gov, thank you for providing the script! I’m wondering if there’s a way to automate the process of pulling the necessary Python versions beforehand. Do you have any automated solution for this, and if so, how do you implement it?

Currently, I'm using uv tool run nox -s tests, but it doesn’t automatically pull the required Python binaries when executed. I will have to install them first or parameterize the tests by injecting uv python install ... into every run. I could enforce developers to manually specify an array of Python distributions in something like tests.py file then install all pythons before executing nox: uv run tests.py, but that introduces an extra step that I’d prefer to avoid.

From my understanding, to utilize the script you provided, I would first need to identify the specific Python versions I want, then have uv install them, and only afterward add them to the PATH. Is there a more simpler way around? Or, rather, a more standardized way that I could pass down onto users?

[henryiii]

nox is supposed to install Python automatically as needed as long as you are using the uv backend. That was added in wntrblm/nox#842, which was released almost exactly a month ago.

[Levelleor]

Hey, this is great news actually! Just to make it clear, were you talking about setting venv backend to uv? @henryiii

nox --default-venv-backend uv ...

This worked for me.

[henryiii]

Yes, or

nox.options.default_venv_backend = "uv|virtualenv"

(I put this in all my noxfiles)

[FCamborda]

For anybody struggling with this, make sure that you are using uv >= 0.4.16. We were on 0.4.4 and this line was being skipped:
https://github.com/saucoide/nox/blob/1e01f798facd6b8b3e44cae6389e3319afe02016/nox/virtualenv.py#L564
But now it works like a charm!!