Having a .python-version or .python-versions file breaks Python version definition in Nox with uv backend

[MicaelJarniac]

Current Behavior

If there is a .python-version or .python-versions file in the project's root, uv will default to that after a uv sync, and ignore the Python version specified in the Noxfile.

Expected Behavior

It should ignore these files and use the Python version defined by Nox.

Steps To Reproduce

1. uv init nox-uv
2. code nox-uv
3. uv add --dev "nox[uv]"
4. uv sync
5. Create the following noxfile.py in the root:

import nox

nox.options.default_venv_backend = "uv"


@nox.session(python=["3.8", "3.9"])
def py_ver(session: nox.Session) -> None:
    session.run_install(
        "uv",
        "sync",
        "--no-group=dev",
        env={"UV_PROJECT_ENVIRONMENT": session.virtualenv.location},
    )
    session.run("python", "--version")

6. nox
7. Notice how it'll run all sessions with Python 3.13.

❯ nox
nox > Running session py_ver-3.8
nox > Creating virtual environment (uv) using python3.8 in .nox/py_ver-3-8
nox > uv sync --no-group=dev
Using CPython 3.13.2
Removed virtual environment at: .nox/py_ver-3-8
Creating virtual environment at: .nox/py_ver-3-8
Resolved 13 packages in 0.92ms
Audited in 0.01ms
nox > python --version
Python 3.13.2
nox > Session py_ver-3.8 was successful.
nox > Running session py_ver-3.9
nox > Creating virtual environment (uv) using python3.9 in .nox/py_ver-3-9
nox > uv sync --no-group=dev
Using CPython 3.13.2
Removed virtual environment at: .nox/py_ver-3-9
Creating virtual environment at: .nox/py_ver-3-9
Resolved 13 packages in 1ms
Audited in 0.01ms
nox > python --version
Python 3.13.2
nox > Session py_ver-3.9 was successful.
nox > Ran multiple sessions:
nox > * py_ver-3.8: success
nox > * py_ver-3.9: success

Environment

- OS: Ubuntu 24.04
- Python: 3.13
- Nox: 2025.2.9

Anything else?

I'm following the official Nox recipe for uv found here:
https://nox.thea.codes/en/stable/cookbook.html#using-a-lockfile

I believe this may be solved by adapting the recipe.

[henryiii]

You can add --no-config, though that will also ignore uv.toml. Thanks to @zanieb for the suggestion! We might want to add that to the recipe for now until there's a more fine-grained way to avoid replacing the venv.

[MicaelJarniac]

--no-config didn't seem to work.

❯ nox
nox > Running session py_ver-3.8
nox > Creating virtual environment (uv) using python3.8 in .nox/py_ver-3-8
nox > uv sync --no-group=dev --no-config
Using CPython 3.13.2
Removed virtual environment at: .nox/py_ver-3-8
Creating virtual environment at: .nox/py_ver-3-8
Resolved 13 packages in 1ms
Audited in 0.02ms
nox > python --version
Python 3.13.2
nox > Session py_ver-3.8 was successful.
nox > Running session py_ver-3.9
nox > Creating virtual environment (uv) using python3.9 in .nox/py_ver-3-9
nox > uv sync --no-group=dev --no-config
Using CPython 3.13.2
Removed virtual environment at: .nox/py_ver-3-9
Creating virtual environment at: .nox/py_ver-3-9
Resolved 13 packages in 1ms
Audited in 0.02ms
nox > python --version
Python 3.13.2
nox > Session py_ver-3.9 was successful.
nox > Ran multiple sessions:
nox > * py_ver-3.8: success
nox > * py_ver-3.9: success

[henryiii]

Is your requires-python correct? It will ignore the request if it doesn't match that, and uv sets it to >=3.13 by default. I made this mistake the first time, too.

Another way to do it is to set it via -p, like this:

@nox.session(python=["3.8", "3.9"])
def py_ver(session: nox.Session) -> None:
    session.run_install(
        "uv",
        "sync",
        "--no-default-groups",
        f"-p{session.virtualenv.location}",
        env={"UV_PROJECT_ENVIRONMENT": session.virtualenv.location},
    )
    session.run("python", "--version")

[MicaelJarniac]

It was a bad requires-python indeed, thanks!