Nox

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Introduction

Nox offers flexible test automation for Python. Where tox uses declarative configuration files, Nox uses a noxfile.py, where each session is a Python function decorated with @nox.session. That makes conditional logic, loops and shared helpers natural, which many teams find easier to maintain for complex workflows.

Inside a session you call session.install(...) to add dependencies and session.run(...) to execute commands. Sessions can be parameterised by Python version (python=["3.11", "3.12"]) or with @nox.parametrize for dependency matrices. nox -s tests runs one session, nox -l lists sessions, and -R/--reuse-existing-virtualenvs speeds up repeated runs. Nox supports several virtual environment backends, including virtualenv, venv, conda and uv. It uses calendar versioning; release 2026.10.20 came out on 20 October 2026. The project is maintained under the wntrblm organisation.

The source code is at wntrblm/nox on GitHub under the Apache-2.0 licence, with about 1,600 stars; the most recent commit was on 6 November 2026.

Because sessions are code, it is easy to share helper functions between sessions or skip work based on environment variables.

Features

1. Python configuration: Sessions are plain Python functions.
2. Parameterisation: Run sessions across interpreters and dependency versions.
3. Multiple backends: virtualenv, venv, conda and uv.
4. Session selection: Run, list and tag sessions from the CLI.

Pros

1. Free to use: Open source under the Apache-2.0 licence, so the core tool costs nothing to use.
2. Actively maintained: The repository had commits as recent as 6 November 2026.

Cons

1. More code to maintain: Flexibility means noxfiles can grow complex.
2. Smaller plugin ecosystem: Fewer plugins than tox.

Usage and Links (Example)

# noxfile.py
import nox

@nox.session(python=["3.11", "3.12", "3.13"])
def tests(session):
    session.install("-e", ".", "pytest", "coverage")
    session.run("coverage", "run", "-m", "pytest", *session.posargs)

@nox.session
def lint(session):
    session.install("ruff")
    session.run("ruff", "check", "src", "tests")

pip install nox
nox -s tests
nox -s lint

Resources:

• Nox website
• GitHub repository

Pricing

Nox is free and open source under the Apache-2.0 licence.

Recommended for?

1. Python teams who prefer code over configuration for test automation.
2. Projects with complex, conditional test workflows.

Conclusion

Nox brings the power of Python to test automation, a strong alternative to tox when flexibility matters.