Getting Started

This tutorial walks you through creating your first tox project from scratch. By the end, you will have a working tox configuration that runs tests and linting across multiple Python versions.

Prerequisites

Before starting, make sure you have:

  • Python 3.10 or later installed

  • tox installed (see Installation)

  • A Python project you want to test (or follow along to create one)

Verify tox is available:

tox --version

Creating your first configuration

tox needs a configuration file where you define what tools to run and how to set up environments for them. tox supports two configuration formats: TOML and INI. TOML is the recommended format for new projects – it is more robust, has proper type support, and avoids ambiguities inherent in INI parsing. INI remains supported for existing projects.

Create a tox.toml (or tox.ini) at the root of your project:

env_list = ["3.13", "3.12", "lint"]

[env_run_base]
description = "run the test suite with pytest"
deps = [
    "pytest>=8",
]
commands = [["pytest", { replace = "posargs", default = ["tests"], extend = true }]]

[env.lint]
description = "run linters"
skip_install = true
deps = ["ruff"]
commands = [["ruff", "check", { replace = "posargs", default = ["."], extend = true }]]

[tox]
env_list = 3.13, 3.12, lint

[testenv]
description = run the test suite with pytest
deps =
    pytest>=8
commands =
    pytest {posargs:tests}

[testenv:lint]
description = run linters
skip_install = true
deps =
    ruff
commands = ruff check {posargs:.}

Tip

You can also generate a tox.ini file automatically by running tox quickstart and answering a few questions.

Understanding the configuration

The configuration has two parts: core settings and environment settings.

Core settings

Core settings affect all environments or configure how tox itself behaves. They live at the root level in tox.toml (or under the [tox] section in tox.ini).

env_list = ["3.13", "3.12", "lint"]

[tox]
env_list = 3.13, 3.12, lint

The env_list setting defines which environments run by default when you invoke tox without specifying any. Both formats support generating environment matrices from factor combinations — INI uses curly-brace expansion (3.{10-}), while TOML uses product dicts ({ product = [{ prefix = "py3", start = 10 }, ["django42"]] }). See Generative environment list for details. For the full list of core options, see Core.

Environment settings

Each tox environment has its own configuration. Settings defined at the base level (env_run_base in TOML, testenv in INI) are inherited by all environments unless overridden. Individual environments are configured under env.<name> in TOML or testenv:<name> in INI.

[env_run_base]
description = "run the test suite with pytest"
deps = ["pytest>=8"]
commands = [["pytest", { replace = "posargs", default = ["tests"], extend = true }]]

[env.lint]
description = "run linters"
skip_install = true
deps = ["ruff"]
commands = [["ruff", "check", "."]]

[testenv]
description = run the test suite with pytest
deps =
    pytest>=8
commands =
    pytest {posargs:tests}

[testenv:lint]
description = run linters
skip_install = true
deps =
    ruff
commands = ruff check .

Here the lint environment overrides the base settings entirely, while 3.13 and 3.12 inherit from the base.

Tip

Options must go in the correct section — placing a core option in an environment section (or vice versa) silently has no effect. Run tox run -v or tox config to check for misplaced keys.

Environment names and Python versions

Environment names can consist of alphanumeric characters, dashes, and dots. Names are split on dashes into factors – for example py311-django42 splits into factors py311 and django42. Additionally, the current platform (like linux, darwin, win32) is automatically available as an implicit factor for conditional configuration.

tox recognizes certain naming patterns and automatically sets the Python interpreter:

  • N.M: CPython N.M (e.g. 3.13) – preferred

  • pyNM or pyN.M: CPython N.M (e.g. py313 or py3.13) – legacy, still supported

  • pypyNM: PyPy N.M

  • cpythonNM: CPython N.M

  • graalpyNM: GraalPy N.M

Prefer the N.M form (e.g. 3.14) over pyNMM (e.g. py314). The dotted form is unambiguous, reads more naturally in environment lists and CI output, and avoids confusion for Python versions >= 3.10 where the concatenated digits become three characters.

Note

Version-number factors like 3.14 can be checked in conditional expressions using subscript syntax: factor['3.14'] (the dot-syntax factor.3.14 would be a syntax error). See Conditional value reference for details.

If the name doesn’t match any pattern, tox uses the same Python as the one tox is installed into (this is the case for lint in our example). To override this fallback, set default_base_python:

[env_run_base]
default_base_python = ["3.14", "3.13"]

This pins a default Python version for environments without a Python factor, improving reproducibility across machines with different system Pythons.

Tip

If your project uses PEP 751 lock files (pylock.toml), you can install locked dependencies via pylock instead of listing packages in deps.

For the full list of environment options, see tox environment.

Scaling to multiple Python versions

When your test matrix grows beyond a few environments, use env_base sections to define named templates that generate environments from factor combinations:

[env_base.test]
factors = [["3.13", "3.14"]]
deps = ["pytest>=8"]
commands = [["pytest"]]

This generates test-3.13 and test-3.14, each inheriting deps and commands from the template. The template itself inherits from env_run_base, so global defaults still apply. See Environment base templates for details.

Running your environments

Run all default environments (those listed in env_list):

tox

Run a specific environment:

tox run -e lint

Run multiple environments:

tox run -e 3.13,lint

Pass extra arguments to the underlying tool using --:

# Run pytest in verbose mode
tox run -e 3.13 -- -v

# Run ruff on a specific file
tox run -e lint -- src/mymodule.py

The { replace = "posargs" } in TOML (or {posargs} in INI) is a placeholder that gets replaced by whatever you pass after --.

Understanding the output

On the first run, tox creates virtual environments and installs dependencies. Subsequent runs reuse existing environments unless dependencies change:

$ tox run -e 3.13,lint
3.13: install_deps> python -m pip install 'pytest>=8'
3.13: commands[0]> pytest tests
========================= 3 passed in 0.12s =========================
3.13: OK ✔ in 5.43s
lint: install_deps> python -m pip install ruff
lint: commands[0]> ruff check .
All checks passed!
lint: OK ✔ in 2.11s

  3.13: OK (5.43=setup[3.21]+cmd[2.22] seconds)
  lint: OK (2.11=setup[1.05]+cmd[1.06] seconds)
  congratulations :)

tox will automatically detect changes to your dependencies and recreate environments when needed. You can force a full recreation with the -r flag:

tox run -e 3.13 -r

If tools inside the environment maintain their own caches (e.g. pre-commit), you can use recreate_commands to clean them before the environment directory is removed. See Clean external caches during recreation for details.

If you want to rerun tests without reinstalling dependencies or the package (e.g. when working offline or when nothing has changed), use --skip-env-install:

tox run -e 3.13 --skip-env-install

Listing available environments

See all configured environments and their descriptions:

$ tox list
default environments:
3.13 -> run the test suite with pytest
3.12 -> run the test suite with pytest
lint -> run linters

Inspecting configuration

View the resolved configuration for an environment:

tox config -e 3.13 -k deps commands

The output format can be changed with --format and written to a file with -o:

tox config -e 3.13 --format json -o config.json
tox config -e 3.13 --format toml -o config.toml

This is useful for debugging configuration issues and for programmatic consumption.

Next steps

Now that you have a working tox setup, explore these topics: