Use tox to create isolated Python environments, install the dependencies each environment needs, and run your project’s tests across the Python versions you choose to support. With tox 4, a new setup can live in tox.toml; run tox to test the default environment list, or select environments explicitly with tox run -e.
Set up tox 4 with a TOML configuration
The tox documentation project describes tox as a tool that creates virtual environments for multiple Python versions, installs project dependencies, and runs tests in each environment. The example below configures pytest for two Python versions. The version numbers are illustrative; choose interpreters that match your project’s supported Python versions and are available on your machine.
Create tox.toml in your project root:
env_list = ["3.13", "3.12"]
[env_run_base]
deps = ["pytest>=8"]
commands = [["pytest", { replace = "posargs", default = ["tests"], extend = true }]]This sets the default environment list, installs pytest in each environment, and runs pytest against tests by default. The posargs replacement allows arguments supplied after -- on the tox command line to reach pytest.
Current tox documentation recommends TOML for new configuration: use tox.toml, or put the configuration under [tool.tox] in pyproject.toml. The documentation marks tox.ini and setup.cfg configuration as deprecated. If you maintain a project using one of those formats, consult the current tox 4.61.4 Getting Started guide before changing its configuration.
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Install tox and prepare the project
Install tox using the method appropriate to your development environment, then make sure the Python interpreters named in env_list are available. This tutorial does not prescribe a particular installer or operating-system setup; the tox documentation’s Getting Started guide covers the initial setup.
- Put the configuration in the project root. Save the example as
tox.toml, beside the project’s source and tests. - Check the default test path. The example expects tests under
tests. Change the default in the command if your project uses another path. - Ignore tox’s working directory in version control. By default, tox creates virtual environments and stores them in
.toxbeside the configuration. Add.toxto your version-control ignore rules if it is not already ignored. - Run tox from the project root. tox creates the environments and installs the configured dependencies on the first run.
Run all or selected test environments
With the example configuration, these commands cover the most common choices:
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| Goal | Command |
|---|---|
Run every environment in env_list |
tox |
| Run only Python 3.13 | tox run -e 3.13 |
Run Python 3.13 and a configured lint environment |
tox run -e 3.13,lint |
| Pass pytest’s verbose flag through tox | tox run -e 3.13 -- -v |
The multi-environment example works when lint is configured in your tox setup. In the final command, -- separates tox’s options from pytest’s arguments; the posargs placeholder forwards -v to pytest. tox 4 documents environment selection and argument passing in its usage guide.
Understand environment reuse and installation
After the first run, tox normally reuses its environments. It refreshes them when dependencies change, but you can explicitly recreate an environment when you suspect it is stale:
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tox run -e 3.13 -rUse --skip-env-install only when you deliberately want tox to run in an already prepared environment without performing installation. For example, it can be useful for a rerun when dependencies are already installed, including an offline scenario:
tox run -e 3.13 --skip-env-installThis option does not prepare missing dependencies. If the environment is new, incomplete, or out of date, use a normal run or recreate it instead. The tox usage guide documents these run options.
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Run environments in parallel safely
Parallel execution can run selected environments together, which may reduce the elapsed time compared with running them sequentially. Keep sequential runs when they are easier to reason about, or when tests share state. In parallel pytest runs, give each environment its own temporary directory so simultaneous processes do not collide.
Add --basetemp={env_tmp_dir} to the pytest command in tox.toml:
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env_list = ["3.13", "3.12"]
[env_run_base]
deps = ["pytest>=8"]
commands = [["pytest", "--basetemp={env_tmp_dir}", { replace = "posargs", default = ["tests"], extend = true }]]Then run the selected environments in parallel:
tox parallel -e 3.13,3.12Use a separate temporary directory whenever concurrent test runs might otherwise write to the same location. The parallel command and temporary-directory pattern are documented in tox’s usage guide.
Inspect tox configuration and troubleshoot failures
Start by checking what tox resolved, then examine the environment that failed. These commands help distinguish a configuration issue from a test or environment issue:
- List configured environments:
tox list. - Inspect resolved settings:
tox config -e 3.13 -k deps commands. - Show more run detail:
tox run -e 3.13 -vv. - Review environment logs: inspect files under
.tox/<env_name>/log/, replacing<env_name>with the actual environment name. - Check the environment interactively:
tox exec -e 3.13 -- pythonortox exec -e 3.13 -- pip list.
If the environment appears stale, recreate it with tox run -e 3.13 -r. The current tox command reference documents the CLI commands; the usage guide provides workflow guidance.
Common causes and fixes
- A selected environment unexpectedly succeeds: tox can, by default, run an unconfigured environment name using defaults, so a typo may not fail as expected. Run
tox listand inspect the resolved configuration withtox config. - pytest cannot find tests: the example defaults to
tests. Confirm the directory exists or change the default test path in the command. - pytest does not receive a flag: put tox options before
--and pytest options after it, and ensure the configured command includes theposargsreplacement. - Parallel runs interfere with one another: configure
--basetemp={env_tmp_dir}so each environment uses its own temporary directory. - Dependencies or tools appear missing: a run using
--skip-env-installwill not install them. Run tox normally to install dependencies, or recreate the environment if it is stale.
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