Command-line usage


Nox is normally invoked on the command line:


You can also invoke Nox via the Python interpreter:

python3 -m nox

Listing available sessions

To list all available sessions, including parametrized sessions:

nox -l
nox --list
nox --list-sessions

If you’d like to use the output in later processing, you can add --json to get json output for the selected session. Fields include session (pretty name), name, description, python (null if not specified), tags, and call_spec (for parametrized sessions).

Running all sessions

You can run every session by just executing nox without any arguments:


The order that sessions are executed is the order that they appear in the Noxfile.

Specifying one or more sessions

By default Nox will run all sessions defined in the Noxfile. However, you can choose to run a particular set of them using --session, -s, or -e:

nox --session tests
nox -s lint tests
nox -e lint

Nox will run these sessions in the same order they are specified.

If you have a configured session’s virtualenv, you can choose to run only sessions with given Python versions:

nox --python 3.8
nox -p 3.7 3.8

You can also use pytest-style keywords using -k or --keywords, and tags using -t or --tags to filter test sessions:

nox -k "not lint"
nox -k "tests and not lint"
nox -k "not my_tag"
nox -t "my_tag" "my_other_tag"

Specifying parametrized sessions

If you have a parametrized session such as:

@nox.parametrize('django', ['1.9', '2.0'])
def tests(session, django):

Then running nox --session tests will actually run all parametrized versions of the session. If you want the run the session with a particular set of parametrized arguments, you can specify them with the session name:

nox --session "tests(django='1.9')"
nox --session "tests(django='2.0')"

Changing the sessions default backend

By default Nox uses virtualenv as the virtual environment backend for the sessions, but it also supports uv, conda, mamba, micromamba, and venv as well as no backend (passthrough to whatever python environment Nox is running on). You can change the default behaviour by using -db <backend> or --default-venv-backend <backend>. Supported names are ('none', 'uv', 'virtualenv', 'conda', 'mamba', 'venv').

nox -db conda
nox --default-venv-backend conda


The uv, conda, mamba, and micromamba backends require their respective programs be pre-installed. uv is distributed as a Python package and can be installed with the nox[uv] extra.

You can also set this option with the NOX_DEFAULT_VENV_BACKEND environment variable, or in the Noxfile with nox.options.default_venv_backend. In case more than one is provided, the command line argument overrides the environment variable, which in turn overrides the Noxfile configuration.

Note that using this option does not change the backend for sessions where venv_backend is explicitly set.


The uv backend does not install anything by default, including pip, as uv pip is used to install programs instead. If you need to manually interact with pip, you should install it with session.install("pip").

Backends that could be missing (uv, conda, mamba, and micromamba) can have a fallback using |, such as uv|virtualenv or micromamba|mamba|conda. This will use the first item that is available on the users system.

If you need to check to see which backend was selected, you can access it via session.venv_backend in your noxfile.

Forcing the sessions backend

You might work in a different environment than a project’s default continuous integration settings, and might wish to get a quick way to execute the same tasks but on a different venv backend. For this purpose, you can temporarily force the backend used by all sessions in the current Nox execution by using -fb <backend> or --force-venv-backend <backend>. No exceptions are made, the backend will be forced for all sessions run whatever the other options values and Noxfile configuration. Supported names are ('none', 'uv', 'virtualenv', 'conda', 'mamba', 'micromamba', 'venv').

nox -fb conda
nox --force-venv-backend conda

You can also set this option in the Noxfile with nox.options.force_venv_backend. In case both are provided, the commandline argument takes precedence.

Finally note that the --no-venv flag is a shortcut for --force-venv-backend none and allows to temporarily run all selected sessions on the current python interpreter (the one running Nox).

nox --no-venv

Re-using virtualenvs

By default, Nox deletes and recreates virtualenvs every time it is run. This is usually fine for most projects and continuous integration environments as pip’s caching makes re-install rather quick. However, there are some situations where it is advantageous to reuse the virtualenvs between runs. Use -r or --reuse-existing-virtualenvs or for fine-grained control use --reuse-venv=yes|no|always|never:

nox -r
nox --reuse-existing-virtualenvs
nox --reuse-venv=yes # preferred

If the Noxfile sets nox.options.reuse_existing_virtualenvs, you can override the Noxfile setting from the command line by using --no-reuse-existing-virtualenvs. Similarly you can override nox.options.reuse_venvs from the Noxfile via the command line by using --reuse-venv=yes|no|always|never.


--reuse-existing-virtualenvs is a alias for --reuse-venv=yes and --no-reuse-existing-virtualenvs is an alias for --reuse-venv=no.

Additionally, you can skip the re-installation of packages when a virtualenv is reused. Use -R or --reuse-existing-virtualenvs --no-install or --reuse-venv=yes --no-install:

nox -R
nox --reuse-existing-virtualenvs --no-install
nox --reuse-venv=yes --no-install

The --no-install option causes the following session methods to return early:

The never and always options in --reuse-venv gives you more fine-grained control as it ignores when a @nox.session has reuse_venv=True|False defined.

These options have no effect if the virtualenv is not being reused.

Running additional Python versions

In addition to Nox supporting executing single sessions, it also supports running Python versions that aren’t specified using --extra-pythons.

nox --extra-pythons 3.8 3.9 3.10

This will, in addition to specified Python versions in the Noxfile, also create sessions for the specified versions.

This option can be combined with --python to replace, instead of appending, the Python interpreter for a given session:

nox --python 3.11 --extra-python 3.11 -s lint

Instead of passing both options, you can use the --force-python shorthand:

nox --force-python 3.11 -s lint

Also, you can specify python in place of a specific version. This will run the session using the python specified for the current PATH:

nox --force-python python -s lint

Stopping if any session fails

By default Nox will continue to run all sessions even if one fails. You can use --stop-on-first-error to make Nox abort as soon as the first session fails:

nox --stop-on-first-error

If the Noxfile sets nox.options.stop_on_first_error, you can override the Noxfile setting from the command line by using --no-stop-on-first-error.

Failing sessions when the interpreter is missing

By default, when not on CI, Nox will skip sessions where the Python interpreter can’t be found. If you want Nox to mark these sessions as failed, you can use --error-on-missing-interpreters:

nox --error-on-missing-interpreters

If the Noxfile sets nox.options.error_on_missing_interpreters, you can override the Noxfile setting from the command line by using --no-error-on-missing-interpreters.

If being run on Continuous Integration (CI) systems, Nox will treat missing interpreters as errors by default to avoid sessions silently passing when the requested python interpreter is not installed. Nox does this by looking for an environment variable called CI which is a convention used by most CI providers.

Disallowing external programs

By default Nox will warn but ultimately allow you to run programs not installed in the session’s virtualenv. You can use --error-on-external-run to make Nox fail the session if it uses any external program without explicitly passing external=True into

nox --error-on-external-run

If the Noxfile sets nox.options.error_on_external_run, you can override the Noxfile setting from the command line by using --no-error-on-external-run.

Specifying a different configuration file

If for some reason your Noxfile is not named, you can use --noxfile or -f:

nox --noxfile
nox -f

Storing virtualenvs in a different directory

By default Nox stores virtualenvs in ./.nox, however, you can change this using --envdir:

nox --envdir /tmp/envs

Skipping everything but install commands

There are a couple of cases where it makes sense to have Nox only run install commands, such as preparing an environment for offline testing or re-creating the same virtualenvs used for testing. You can use --install-only to skip run commands.

For example, given this Noxfile:

def tests(session):


nox --install-only

Would run both install commands, but skip the run command:

nox > Running session tests
nox > Creating virtualenv using python3.7 in ./.nox/tests
nox > python -m pip install pytest
nox > python -m pip install .
nox > Skipping pytest run, as --install-only is set.
nox > Session tests was successful.

Forcing non-interactive behavior

session.interactive can be used to tell if Nox is being run from an interactive terminal (such as an actual human running it on their computer) vs run in a non-interactive terminal (such as a continuous integration system).

def docs(session):

    if session.interactive:"sphinx-autobuild", ...)
    else:"sphinx-build", ...)

Sometimes it’s useful to force Nox to see the session as non-interactive. You can use the --non-interactive argument to do this:

nox --non-interactive

This will cause session.interactive to always return False.

Controlling color output

By default, Nox will output colorful logs if you’re using in an interactive terminal. However, if you are redirecting stderr to a file or otherwise not using an interactive terminal, or the environment variable NO_COLOR is set, Nox will output in plaintext. If this is not set, and FORCE_COLOR is present, color will be forced.

You can manually control Nox’s output using the --nocolor and --forcecolor flags.

For example, this will always output colorful logs:

nox --forcecolor

However, this will never output colorful logs:

nox --nocolor

Controlling commands verbosity

By default, Nox will only show output of commands that fail, or, when the commands get passed silent=False. By passing --verbose to Nox, all output of all commands run is shown, regardless of the silent argument.

Outputting a machine-readable report

You can output a report in json format by specifying --report:

nox --report status.json

Converting from tox

Nox has experimental support for converting tox.ini files into files. This doesn’t support every feature of tox and is intended to just do most of the mechanical work of converting over- you’ll likely still need to make a few changes to the converted

To use the converter, install nox with the tox_to_nox extra:

pip install --upgrade nox[tox_to_nox]

Then, just run tox-to-nox in the directory where your tox.ini resides:


This will create a based on the environments in your tox.ini. Some things to note:

  • Generative environments work, but will be converted as individual environments. tox-to-nox isn’t quite smart enough to turn these into parametrized sessions, but it should be straightforward to manually pull out common configuration for parametrization.

  • Due to the way tox parses its configuration, all substitutions are baked in when converting. This means you’ll need to replace the static strings in the with appropriate variables.

  • Several non-common tox options aren’t implemented, but it’s possible to do so. Please file a feature request if you run into one you think will be useful.

Shell Completion

Add the appropriate command to your shell’s config file so that it is run on startup. You will likely have to restart or re-login for the autocompletion to start working.


eval "$(register-python-argcomplete nox)"


# To activate completions for zsh you need to have
# bashcompinit enabled in zsh:
autoload -U bashcompinit

# Afterwards you can enable completion for Nox:
eval "$(register-python-argcomplete nox)"


eval `register-python-argcomplete --shell tcsh nox`


register-python-argcomplete --shell fish nox | .