tox

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Introduction

tox describes itself as a command-line-driven CI frontend and development task automation tool. It creates isolated virtual environments, installs your package and its test dependencies into each, and runs the commands you define, so you can verify that a library works on Python 3.10, 3.11, 3.12 and newer, or with different versions of a dependency, using one command.

Configuration lives in tox.ini, setup.cfg or pyproject.toml (and, in tox 4, tox.toml). Each environment has dependencies and commands, and factor syntax lets you build matrices concisely (for example py{311,312}-django{42,50}). tox -e lint runs one environment, tox p runs environments in parallel, and the plugin system extends it; the popular tox-uv plugin uses uv for much faster environment creation. Because the same config runs locally and in CI, tox reduces “works on my machine” differences. It is maintained by the tox-dev organisation, and version 4.67.13 was released on 6 November 2026.

The source code is at tox-dev/tox on GitHub under the MIT licence, with about 4,000 stars; the most recent commit was on 11 November 2026.

Features

1. Isolated environments: Fresh virtualenvs per test environment.
2. Version matrices: Test multiple Pythons and dependency versions with factors.
3. Parallel runs: tox p runs environments concurrently.
4. Plugins: Extend with tox-uv, tox-gh-actions and others.

Pros

1. Free to use: Open source under the MIT licence, so the core tool costs nothing to use.
2. Actively maintained: The repository had commits as recent as 11 November 2026.
3. Community: About 4,000 GitHub stars.

Cons

1. INI-style configuration: Complex matrices can become hard to read; nox uses Python instead.
2. Environment creation cost: Fresh environments take time without caching or tox-uv.

Usage and Links (Example)

# tox.ini
[tox]
env_list = py{311,312,313}, lint

[testenv]
deps = pytest
       coverage
commands = coverage run -m pytest {posargs}

[testenv:lint]
deps = ruff
commands = ruff check src tests

pip install tox
tox            # all environments
tox -e py312   # one environment
tox p          # in parallel

Resources:

• tox website
• GitHub repository

Pricing

tox is free and open source under the MIT licence.

Recommended for?

1. Python library maintainers supporting several Python versions.
2. Teams wanting identical test commands locally and in CI.

Conclusion

tox is the long-standing standard for Python test environment automation, especially valuable for libraries with broad version support.