session.install fails with uv backend if UV_PYTHON is set

[MikkelSchubert]

Current Behavior

Session.install fails when using uv as the backend, and the UV_PYTHON environment variable is set to a different version of Python than is being used by the current session

Expected Behavior

nox could perhaps filter UV_PYTHON and/or show a warning if it is set

Steps To Reproduce

For the following nox file

import nox

nox.options.default_venv_backend = "uv"

@nox.session(python=["3.11"])
def tests(session):
    session.install("pytest")

Without UV_PYTHON

$ rm -rf .nox __pycache__
$ unset UV_PYTHON
$ nox
nox > Running session tests-3.11
nox > Creating virtual environment (uv) using python3.11 in .nox/tests-3-11
nox > uv pip install pytest
nox > Session tests-3.11 was successful.

With UV_PYTHON

$ rm -rf .nox __pycache__
$ export UV_PYTHON=3.12
$ nox
nox > Running session tests-3.11
nox > Creating virtual environment (uv) using python3.11 in .nox/tests-3-11
nox > uv pip install pytest
nox > Command uv pip install pytest failed with exit code 2:
error: No virtual environment found for Python 3.12; run `uv venv` to create an environment, or pass `--system` to install into a non-virtual environment
nox > Session tests-3.11 failed.

Environment

- OS: Linux Mint 22
- Python: 3.11.11 (managed by uv) and 3.12.3 (system)
- Nox: 2025.2.9
- Uv: 0.6.11

Anything else?

No response

[johnthagen]

Related

• docs: update uv recipe #933