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Use tox 4 to create isolated Python environments, install test dependencies, and run your project’s tests across the Python versions you support. For a new setup, put the configuration in tox.toml, define the environments and test command, then run tox.
What tox does—and what it does not do
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.” See the tox command reference. tox automates the test workflow you configure; it does not determine whether your tests are sufficient or guarantee software quality.
This guide follows the tox 4 configuration style documented in the tox 4.61.4 Getting Started guide and the stable usage guide. Python version numbers in examples are illustrative: choose versions that match your project’s support policy and interpreters available on your machine or CI runner.
Configure a basic pytest project with tox 4
Use tox.toml for a new configuration
Create tox.toml in the project root:
env_list = ["3.13", "3.12"]
[env_run_base]
deps = ["pytest>=8"]
commands = [["pytest", { replace = "posargs", default = ["tests"], extend = true }]]env_list sets the default environments that a plain tox run executes. The shared env_run_base settings give each listed environment pytest and the same test command. The posargs replacement makes arguments supplied after -- available to pytest, while default = ["tests"] runs that directory when no extra arguments are supplied.
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tox must be installed and callable as tox in your shell; the Python interpreters named by your environment list must also be available. If you use a project-specific dependency manager or build configuration, check the tox getting-started guide for the applicable setup rather than assuming this small example covers every packaging layout.
Where to put configuration
For new projects, the current documentation recommends TOML: use tox.toml as the primary configuration file, or put settings in pyproject.toml under [tool.tox]. The documentation marks tox.ini and setup.cfg as deprecated. Existing projects can maintain their current configuration while planning a deliberate migration; do not mix formats without checking tox’s configuration precedence and behavior in the version you use.
Run all environments or select specific ones
Run the default environment list
From the directory containing the tox configuration, run:
toxtox creates the configured virtual environments as needed, installs their dependencies, and runs the commands. Its default environment storage location is .tox beside the configuration; make sure .tox is excluded from version control if it is not already ignored.
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Run one or several environments
Use -e to select environments instead of running the full default list:
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tox run -e 3.13
tox run -e 3.13,lintThe second example assumes an environment named lint is configured. Environment names should match the project’s actual configuration and interpreter setup.
One tox behavior can disguise a typo: an unconfigured environment name may still run with defaults. If a selection unexpectedly succeeds, inspect the configured names with tox list and review resolved settings rather than assuming tox validated the name against your intended matrix.
Pass pytest options
Put pytest arguments after the separator --:
tox run -e 3.13 -- -v
tox run -e 3.13 -- tests/test_api.py -qBecause the command uses the posargs replacement, these options and paths are forwarded to pytest. Without a supplied path, the configured default is tests.
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On the first run, tox creates each environment and installs its dependencies. Later runs reuse prepared environments unless dependency changes require an update. This makes repeat runs more convenient, but it also means an old or damaged environment can be relevant when results seem inconsistent.
Recreate an environment
Force tox to recreate a selected environment when you need a clean setup:
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tox run -e 3.13 -rRecreation entails rebuilding that environment and reinstalling dependencies, so it is slower than a normal reuse run.
Skip installation only when reuse is intentional
If the environment is already prepared and you specifically want to rerun without installation—for example, in a deliberate offline workflow—use:
tox run -e 3.13 --skip-env-installThis option does not prepare missing dependencies. Use a normal tox run when the environment needs setup or dependency changes to be applied.
Run selected environments in parallel safely
Parallel execution can run selected environments together:
tox parallel -e 3.13,3.12Tests that write to a shared temporary directory can interfere with each other when run concurrently. Isolate pytest’s temporary base directory by adding --basetemp={env_tmp_dir} to the configured command. For the earlier example, the command setting becomes:
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commands = [["pytest", "--basetemp={env_tmp_dir}", { replace = "posargs", default = ["tests"], extend = true }]]Use sequential execution when your tests or fixtures rely on shared paths or other shared state that is not isolated. Parallel execution is an execution choice, not a guarantee that every test suite is safe to run concurrently.
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Check what tox resolved
List configured environments and inspect the selected environment’s effective settings:
tox list
tox config -e 3.13 -k deps commandsThis helps distinguish a configuration issue—such as the wrong command or dependencies—from a test failure.
Increase verbosity and inspect logs
Rerun with more detail, then review the environment logs:
tox run -e 3.13 -vvtox stores logs under .tox/<env_name>/log/. Use the relevant environment’s log to locate the command and failure details.
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Inspect the prepared environment
Use tox exec to run a diagnostic command inside the selected environment:
tox exec -e 3.13 -- python
tox exec -e 3.13 -- pip listThe first command opens Python in that environment; the second lists its installed packages. If the environment appears stale or inconsistent, recreate it with tox run -e 3.13 -r.
Common symptoms and fixes
- The requested Python environment cannot be created: verify the corresponding interpreter is installed and available to tox, and confirm the environment name matches the configuration.
- pytest is not found or dependencies are missing: check the resolved
depswithtox configand run without--skip-env-installso tox can install dependencies. - Arguments after
--do not affect pytest: confirm the configured command includes theposargsreplacement. - Parallel test runs collide over temporary files: give each environment its own pytest base directory using
--basetemp={env_tmp_dir}, or run sequentially. - A misspelled environment appears to work: check
tox listand the resolved configuration; tox can run an unconfigured name with defaults.
Or skip the browser setup
For website screenshots in a Python workflow, ScreenshotNeo provides a one-request API; it is separate from tox and does not replace your test runner. Install requests in the environment where you need to make the request, then use this example (replace the URL and API key):
import requests
r = requests.get(
"https://api.screenshotneo.com/v1/shot",
params={"access_key": "YOUR_API_KEY", "url": "https://stripe.com"},
timeout=90,
)
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Frequently Asked Questions
Can tox run a single test file?
Yes. With the example configuration, pass the file path after `–`, such as `tox run -e 3.13 — tests/test_api.py`.
Does tox replace pytest?
No. tox manages environments and runs the configured command; pytest is the test runner in this example.
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