User Guide


tox is an environment orchestrator. Use it to define how to setup and execute various tools on your projects. The tool can set up environments for and invoke:

  • test runners (such as pytest),

  • linters (e.g., flake8),

  • formatters (for example black or isort),

  • documentation generators (e.g., Sphinx),

  • build and publishing tools (e.g., build with twine),


tox needs a configuration file where you define what tools you need to run and how to provision a test environment for these. The canonical file for this is the tox.ini file. For example:

requires =
env_list = lint, type, py{38,39,310,311}

description = run unit tests
deps =
commands =
    pytest {posargs:tests}

description = run linters
skip_install = true
deps =
commands = black {posargs:.}

description = run type checks
deps =
commands =
    mypy {posargs:src tests}


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

The configuration is split into two type of configuration: core settings are hosted under a core tox section while per run environment settings hosted under testenv and testenv:<env_name> sections.

Core settings

Core settings that affect all test environments or configure how tox itself is invoked are defined under the tox section.

requires =
env_list = lint, type, py{38,39,310,311}

We can use it to specify things such as the minimum version of tox required or the location of the package under test. A list of all supported configuration options for the tox section can be found in the configuration guide.

Test environments

Test environments are defined under the testenv section and individual testenv:<env_name> sections, where <env_name> is the name of a specific environment.

description = run unit tests
deps =
commands =
    pytest {posargs:tests}

description = run linters
skip_install = true
deps =
commands = black {posargs:.}

description = run type checks
deps =
commands =
    mypy {posargs:src tests}

Settings defined in the top-level testenv section are automatically inherited by individual environments unless overridden. Test environment names can consist of alphanumeric characters and dashes; for example: py311-django42. The name will be split on dashes into multiple factors, meaning py311-django42 will be split into two factors: py311 and django42. tox defines a number of default factors, which correspond to various versions and implementations of Python and provide default values for base_python:

  • pyNM: configures basepython = pythonN.M

  • pypyNM: configures basepython = pypyN.M

  • jythonNM: configures basepython = jythonN.M

  • cpythonNM: configures basepython = cpythonN.M

  • ironpythonNM: configures basepython = ironpythonN.M

  • rustpythonNM: configures basepython = rustpythonN.M

You can also specify these factors with a period between the major and minor versions (e.g. pyN.M), without a minor version (e.g. pyN), or without any version information whatsoever (e.g. py)

A list of all supported configuration options for the testenv and testenv:<env_name> sections can be found in the configuration guide.

Basic example

env_list =

description = install black in a virtual environment and invoke it on the current folder
deps = black==22.3.0
skip_install = true
commands = black .

description = install pytest in a virtual environment and invoke it on the tests folder
deps =
commands = pytest tests {posargs}

This example contains a global tox section as well as two test environments. Taking the core section first, we use the env_list setting to indicate that this project has two run environments named format and py310 that should be run by default when tox run is invoked without a specific environment.

The formatting environment and test environment are defined separately via the testenv:format and testenv:py310 sections, respectively. For example to format the project we:

  • add a description (visible when you type tox list into the command line) via the description setting

  • define that it requires the black dependency with version 22.3.0 via the deps setting

  • disable installation of the project under test into the test environment via the skip_install setting - black does not need it installed

  • indicate the commands to be run via the commands setting

For testing the project we use the py310 environment. For this environment we:

  • define a text description of the environment via the description setting

  • specify that we should install pytest v7.0 or later together with the pytest-sugar project via the deps setting

  • indicate the command(s) to be run - in this case pytest tests - via the commands setting

{posargs} is a place holder part for the CLI command that allows us to pass additional flags to the pytest invocation, for example if we’d want to run pytest tests -v as a one off, instead of tox run -e py310 we’d type tox run -e py310 -- -v. The -- delimits flags for the tox tool and what should be forwarded to the tool within.

tox, by default, always creates a fresh virtual environment for every run environment. The Python version to use for a given environment can be controlled via the base_python configuration, however if not set tox will try to use the environment name to determine something sensible: if the name is in the format of pyxy then tox will create an environment with CPython with version x.y (for example py310 means CPython 3.10). If the name does not match this pattern it will use a virtual environment with the same Python version as the one tox is installed into (this is the case for format).

tox environments are reused between runs, so while the first tox run -e py310 will take a while as tox needs to create a virtual environment and install pytest and pytest-sugar in it, subsequent runs only need to reinstall your project, as long as the environments dependency list does not change.

Almost every step and aspect of virtual environments and command execution can be customized. You’ll find an exhaustive list of configuration flags (together with what it does and detailed explanation of what values are accepted) at our configuration page.

System overview

Below is a graphical representation of the tox states and transition pathways between them:


The primary tox states are:

  1. Configuration: load tox configuration files (such as tox.ini, pyproject.toml and and merge it with options from the command line plus the operating system environment variables.

  2. Environment: for each selected tox environment (e.g. py310, format) do:

    1. Creation: create a fresh environment; by default virtualenv is used, but configurable via runner. For virtualenv tox will use the virtualenv discovery logic where the python specification is defined by the tox environments base_python (if not set will default to the environments name). This is created at first run only to be re-used at subsequent runs. If certain aspects of the project change (python version, dependencies removed, etc.), a re-creation of the environment is automatically triggered. To force the recreation tox can be invoked with the recreate flag (-r).

    2. Install dependencies (optional): install the environment dependencies specified inside the deps configuration section, and then the earlier packaged source distribution. By default pip is used to install packages, however one can customize this via install_command. Note pip will not update project dependencies (specified either in the install_requires or the extras section of the if any version already exists in the virtual environment; therefore we recommend to recreate your environments whenever your project dependencies change.

    3. Packaging (optional): create a distribution of the current project.

      1. Build: If the tox environment has a package configured tox will build a package from the current source tree. If multiple tox environments are run and the package built are compatible in between them then it will be reused. This is to ensure that we build the package as rare as needed. By default for Python a source distribution is built as defined via the pyproject.toml style build (see PEP-517 and PEP-518).

      2. Install the package dependencies. If this has not changed since the last run this step will be skipped.

      3. Install the package. This operation will force reinstall the package without its dependencies.

    4. Commands: run the specified commands in the specified order. Whenever the exit code of any of them is not zero, stop and mark the environment failed. When you start a command with a dash character, the exit code will be ignored.

  3. Report print out a report of outcomes for each tox environment:

    ____________________ summary ____________________
    py37: commands succeeded
    ERROR:   py38: commands failed

    Only if all environments ran successfully tox will return exit code 0 (success). In this case you’ll also see the message congratulations :).

tox will take care of environment variable isolation for you. That means it will remove system environment variables not specified via passenv. Furthermore, it will also alter the PATH variable so that your commands resolve within the current active tox environment. In general, all executables outside of the tox environment are available in commands, but external commands need to be explicitly allowed via the allowlist_externals configuration.

Main features

  • automation of tedious Python related test activities

  • test your Python package against many interpreter and dependency configurations

    • automatic customizable (re)creation of virtualenv test environments

    • installs your project into each virtual environment

    • test-tool agnostic: runs pytest, nose or unittest in a uniform manner

  • plugin system to modify tox execution with simple hooks.

  • uses pip and virtualenv by default. Support for plugins replacing it with their own.

  • cross-Python compatible: tox requires CPython 3.7 and higher, but it can create environments 2.7 or later

  • cross-platform: Windows, macOS and Unix style environments

  • full interoperability with devpi: is integrated with and is used for testing in the devpi system, a versatile PyPI index server and release managing tool

  • driven by a simple (but flexible to allow expressing more complicated variants) ini-style config file

  • documented examples and configuration

  • concise reporting about tool invocations and configuration errors

  • supports using different / multiple PyPI index servers


In case the installed tox version does not satisfy either the min_version or the requires, tox will automatically create a virtual environment under provision_tox_env name that satisfies those constraints and delegate all calls to this meta environment. This should allow satisfying constraints on your tox environment automatically, given you have at least version 3.8.0 of tox.

For example given:

min_version = 4
requires = tox-docker>=1

if the user runs it with tox 3.8 or later the installed tox application will automatically ensure that both the minimum version and requires constraints are satisfied, by creating a virtual environment under .tox folder, and then installing into it tox>=4 and tox-docker>=1. Afterwards all tox invocations are forwarded to the tox installed inside .tox\.tox folder (referred to as meta-tox or auto-provisioned tox).

This allows tox to automatically setup itself with all its plugins for the current project. If the host tox satisfies the constraints expressed with the requires and min_version no such provisioning is done (to avoid setup cost and indirection when it’s not explicitly needed).

Cheat sheet

This section details information that you’ll use most often in short form.


  • Each tox subcommand has a 1 (or 2) letter shortcut form too, e.g. tox run can also be written as tox r or tox config can be shortened to tox c.

  • To run all tox environments defined in the env_list run tox without any flags: tox.

  • To run a single tox environment use the -e flag for the run sub-command as in tox run -e py310.

  • To run two or more tox environment pass comma separated values, e.g. tox run -e format,py310. The run command will run the tox environments sequentially, one at a time, in the specified order.

  • To run two or more tox environment in parallel use the parallel sub-command , e.g. tox parallel -e py39,py310. The --parallel flag for this sub-command controls the degree of parallelism.

  • To view the configuration value for a given environment and a given configuration key use the config sub-command with the -k flag to filter for targeted configuration values: tox config -e py310 -k pass_env.

  • tox tries to automatically detect changes to your project dependencies and force a recreation when needed. Unfortunately the detection is not always accurate, and it also won’t detect changes on the PyPI index server. You can force a fresh start for the tox environments by passing the -r flag to your run command. Whenever you see something that should work but fails with some esoteric error it’s recommended to use this flag to make sure you don’t have a stale Python environment; e.g. tox run -e py310 -r would clean the run environment and recreate it from scratch.

Config files

  • Every tox environment has its own configuration section (e.g. in case of tox.ini configuration method the py310 tox environments configuration is read from the testenv:py310 section). If the section is missing or does not contain that configuration value, it will fall back to the section defined by the base configuration (for tox.ini this is the testenv section). For example:

    commands = pytest tests
    description = run the test suite with pytest

    Here the environment description for test is taken from testenv:test. As commands is not specified, the value defined under the testenv section will be used. If the base environment is also missing a configuration value then the configuration default will be used (e.g. in case of the pass_env configuration here).

  • To change the current working directory for the commands run use change_dir (note this will make the change for all install commands too - watch out if you have relative paths in your project dependencies).

  • Environment variables: - To view environment variables set and passed down use tox c -e py310 -k set_env pass_env. - To pass through additional environment variables use pass_env. - To set environment variables use set_env.

  • Setup operation can be configured via the commands_pre, while teardown commands via the commands_post.

  • Configurations may be set conditionally within the tox.ini file. If a line starts with an environment name or names, separated by a comma, followed by : the configuration will only be used if the environment name(s) matches the executed tox environment. For example:

    deps =
       format: black
       py310,py39: pytest

    Here pip will be always installed as the configuration value is not conditional. black is only used for the format environment, while pytest is only installed for the py310 and py39 environments.

Parallel mode

tox allows running environments in parallel mode via the parallel sub-command:

  • After the packaging phase completes tox will run the tox environments in parallel processes (multi-thread based).

  • the --parallel flag takes an argument specifying the degree of parallelization, defaulting to auto:

    • all to run all invoked environments in parallel,

    • auto to limit it to CPU count,

    • or pass an integer to set that limit.

  • Parallel mode displays a progress spinner while running tox environments in parallel, and reports outcome of these as soon as they have been completed with a human readable duration timing attached. To run parallelly without the spinner, you can use the --parallel-no-spinner flag.

  • Parallel mode by default shows output only of failed environments and ones marked as parallel_show_output =True.

  • There’s now a concept of dependency between environments (specified via depends), tox will re-order the environment list to be run to satisfy these dependencies, also for sequential runs. Furthermore, in parallel mode, tox will only schedule a tox environment to run once all of its dependencies have finished (independent of their outcome).


    depends does not pull in dependencies into the run target, for example if you select py310,py39,coverage via the -e tox will only run those three (even if coverage may specify as depends other targets too - such as py310, py39, py38, py37).

  • --parallel-live/-o allows showing the live output of the standard output and error, also turns off reporting as described above.

  • Note: parallel evaluation disables standard input. Use non parallel invocation if you need standard input.

Example final output:

$ tox -e py310,py39,coverage -p all
✔ OK py39 in 9.533 seconds
✔ OK py310 in 9.96 seconds
✔ OK coverage in 2.0 seconds
___________________________ summary ______________________________________________________
  py310: commands succeeded
  py39: commands succeeded
  coverage: commands succeeded
  congratulations :)

Example progress bar, showing a rotating spinner, the number of environments running and their list (limited up to 120 characters):

 [2] py310 | py39


tox always builds projects in a PEP-518 compatible virtual environment and communicates with the build backend according to the interface defined in PEP-517 and PEP-660. To define package build dependencies and specify the build backend to use create a pyproject.toml at the root of the project. For example to use hatch:

build-backend = ""
requires = ["hatchling>=0.22", "hatch-vcs>=0.2"]

By default tox will create and install a source distribution. You can configure to build a wheel instead by setting the package configuration to wheel. Wheels are much faster to install than source distributions.

To query the projects dependencies tox will use a virtual environment whose name is defined under the package_env configuration (by default .pkg). The virtual environment used for building the package depends on the artifact built:

  • for source distribution the package_env,

  • for wheels the name defined under wheel_build_env (this depends on the Python version defined by the target tox environment under base_python, if the environment targets CPython 3.10 it will be .pkg-cpython310 or for PyPy 3.9 it will be .pkg-pypy39).

For pure Python projects (non C-Extension ones) it’s recommended to set wheel_build_env to the same as the package_env. This way you’ll build the wheel once and install the same wheel for all tox environments.

Advanced features

tox supports these features that 90 percent of the time you’ll not need, but are very useful the other ten percent.

Generative environments

Generative environment list

If you have a large matrix of dependencies, python versions and/or environments you can use a generative env_list and conditional settings to express that in a concise form:

env_list = py{311,310,39}-django{41,40}-{sqlite,mysql}

deps =
    django41: Django>=4.1,<4.2
    django40: Django>=4.0,<4.1
    # use PyMySQL if factors "py311" and "mysql" are present in env name
    py311-mysql: PyMySQL
    # use urllib3 if any of "py311" or "py310" are present in env name
    py311,py310: urllib3
    # mocking sqlite on 3.11 and 3.10 if factor "sqlite" is present
    py{311,310}-sqlite: mock

This will generate the following tox environments:

> tox l
default environments:
py311-django41-sqlite -> [no description]
py311-django41-mysql  -> [no description]
py311-django40-sqlite -> [no description]
py311-django40-mysql  -> [no description]
py310-django41-sqlite -> [no description]
py310-django41-mysql  -> [no description]
py310-django40-sqlite -> [no description]
py310-django40-mysql  -> [no description]
py39-django41-sqlite  -> [no description]
py39-django41-mysql   -> [no description]
py39-django40-sqlite  -> [no description]
py39-django40-mysql   -> [no description]

Generative section names

Suppose you have some binary packages, and need to run tests both in 32 and 64 bits. You also want an environment to create your virtual env for the developers.

base_python =
    py311-x86: python3.11-32
    py311-x64: python3.11-64
commands = pytest

envdir =
    x86: .venv-x86
    x64: .venv-x64
> tox l
default environments:
py          -> [no description]

additional environments:
py310-black -> [no description]
py310-lint  -> [no description]
py311-black -> [no description]
py311-lint  -> [no description]

Disallow command line environments which are not explicitly specified in the config file

Previously, any environment would be implicitly created even if no such environment was specified in the configuration file. For example, given this config:

deps = pytest
commands = pytest

Running tox -e unit would run our tests but running tox -e unt or tox -e unti would ultimately succeed without running any tests. A special exception is made for environments starting in py*. In the above example running tox -e py310 would still function as intended.