tox

tox is a generic virtual environment management and test automation tool for Python, created by Holger Krekel (the same developer behind the popular pytest framework). It addresses a common pain point in Python development: ensuring that a package works correctly across multiple Python versions and dependency configurations. tox automates the creation of isolated virtual environments, installation of the package and its dependencies, and execution of test suites, linting, and build steps in each environment. The core workflow is defined in a tox.ini configuration file, where developers specify one or more test environments (called envlist), each with its own Python version, dependencies, and commands to run. tox then creates a virtualenv for each environment, installs the specified dependencies, runs the commands, and reports the results. Key features include: multi-version testing (running tests against Python 3.9, 3.10, 3.11, 3.12, and 3.13 in parallel), dependency specification per environment (different test dependencies for different scenarios like coverage, linting, type checking), parallel execution (tox run-parallel or tox p for running all environments concurrently on multiple CPU cores), plugin architecture for extending functionality (tox-pip-extensions, tox-conda, tox-pyenv, and more), factor-based conditional dependencies (selecting different dependencies based on environment factors like Python version or platform), integration with all major CI/CD systems, and support for packaging workflows (building source distributions and wheels via the tox build command). tox 4.0 introduced a complete rewrite with improved performance, a new plugin API, and better cross-platform support. As of 2026, tox has over 4,400 stars.