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Configuration & API

Noxfile

Nox looks for configuration in a file named noxfile.py by default. You can specify a different file using the --noxfile argument when running nox.

Defining sessions

nox.session

Nox sessions are configured via standard Python functions that are decorated with @nox.session. For example:

import nox

@nox.session
def tests(session):
    session.run('pytest')

You can also configure sessions to run against multiple Python versions as described in virtualenv config and parametrize sessions as described in parametrized sessions <parametrized>.

Session description

You can add a description to your session using a docstring. The first line will be shown when listing the sessions. For example:

import nox

@nox.session
def tests(session):
    """Run the test suite."""
    session.run('pytest')

The nox --list command will show:

$ nox --list
Available sessions:
* tests -> Run the test suite.

Session name

By default Nox uses the decorated function's name as the session name. This works wonderfully for the vast majority of projects, however, if you need to you can customize the session's name by using the name argument to @nox.session. For example:

import nox

@nox.session(name="custom-name")
def a_very_long_function_name(session):
    print("Hello!")

The nox --list command will show:

$ nox --list
Available sessions:
* custom-name

And you can tell nox to run the session using the custom name:

$ nox --session "custom-name"
Hello!

Configuring a session's virtualenv

By default, Nox will create a new virtualenv for each session using the same interpreter that Nox uses. If you installed Nox using Python 3.6, Nox will use Python 3.6 by default for all of your sessions.

You can tell Nox to use a different Python interpreter/version by specifying the python argument (or its alias py) to @nox.session:

@nox.session(python='2.7')
def tests(session):
    pass

Note

The Python binaries on Windows are found via the Python Launcher for Windows (py). For example, Python 3.9 can be found by determining which executable is invoked by py -3.9. If a given test needs to use the 32-bit version of a given Python, then X.Y-32 should be used as the version.

You can also tell Nox to run your session against multiple Python interpreters. Nox will create a separate virtualenv and run the session for each interpreter you specify. For example, this session will run twice - once for Python 2.7 and once for Python 3.6:

@nox.session(python=['2.7', '3.6'])
def tests(session):
    pass

When you provide a version number, Nox automatically prepends python to determine the name of the executable. However, Nox also accepts the full executable name. If you want to test using pypy, for example:

@nox.session(python=['2.7', '3.6', 'pypy-6.0'])
def tests(session):
    pass

When collecting your sessions, Nox will create a separate session for each interpreter. You can see these sessions when running nox --list. For example this Noxfile:

@nox.session(python=['2.7', '3.6', '3.7', '3.8', '3.9'])
def tests(session):
    pass

Will produce these sessions:

* tests-2.7
* tests-3.6
* tests-3.7
* tests-3.8
* tests-3.9

Note that this expansion happens before parameterization occurs, so you can still parametrize sessions with multiple interpreters.

If you want to disable virtualenv creation altogether, you can set python to False, or set venv_backend to "none", both are equivalent. Note that this can be done temporarily through the --no-venv <opt-force-venv-backend> commandline flag, too.

@nox.session(python=False)
def tests(session):
    pass

You can also specify that the virtualenv should always be reused instead of recreated every time:

@nox.session(
    python=['2.7', '3.6'],
    reuse_venv=True)
def tests(session):
    pass

You are not limited to virtualenv, there is a selection of backends you can choose from as venv, conda, mamba, or virtualenv (default):

@nox.session(venv_backend='venv')
def tests(session):
    pass

Finally, custom backend parameters are supported:

@nox.session(venv_params=['--no-download'])
def tests(session):
    pass

Passing arguments into sessions

Often it's useful to pass arguments into your test session. Here's a quick example that demonstrates how to use arguments to run tests against a particular file:

@nox.session
def test(session):
    session.install('pytest')

    if session.posargs:
        test_files = session.posargs
    else:
        test_files = ['test_a.py', 'test_b.py']

    session.run('pytest', *test_files)

Now you if you run:

nox

Then nox will run:

pytest test_a.py test_b.py

But if you run:

nox -- test_c.py

Then nox will run:

pytest test_c.py

Parametrizing sessions

Session arguments can be parametrized with the nox.parametrize decorator. Here's a typical example of parametrizing the Django version to install:

@nox.session
@nox.parametrize('django', ['1.9', '2.0'])
def tests(session, django):
    session.install(f'django=={django}')
    session.run('pytest')

When you run nox, it will create a two distinct sessions:

$ nox
nox > Running session tests(django='1.9')
nox > python -m pip install django==1.9
...
nox > Running session tests(django='2.0')
nox > python -m pip install django==2.0

nox.parametrize has an interface and usage intentionally similar to pytest's parametrize.

nox.parametrize

You can also stack the decorator to produce sessions that are a combination of the arguments, for example:

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

If you run nox --list, you'll see that this generates the following set of sessions:

* tests(database='postgres', django='1.9')
* tests(database='mysql', django='1.9')
* tests(database='postgres', django='2.0')
* tests(database='mysql', django='2.0')

If you only want to run one of the parametrized sessions, see running_paramed_sessions.

Giving friendly names to parametrized sessions

The automatically generated names for parametrized sessions, such as tests(django='1.9', database='postgres'), can be long and unwieldy to work with even with using keyword filtering <opt-sessions-pythons-and-keywords>. You can give parametrized sessions custom IDs to help in this scenario. These two examples are equivalent:

@nox.session
@nox.parametrize('django',
    ['1.9', '2.0'],
    ids=['old', 'new'])
def tests(session, django):
    ...
@nox.session
@nox.parametrize('django', [
    nox.param('1.9', id='old'),
    nox.param('2.0', id='new'),
])
def tests(session, django):
    ...

When running nox --list you'll see their new IDs:

* tests(old)
* tests(new)

And you can run them with nox --sessions "tests(old)" and so on.

This works with stacked parameterizations as well. The IDs are combined during the combination. For example:

@nox.session
@nox.parametrize(
    'django',
    ['1.9', '2.0'],
    ids=["old", "new"])
@nox.parametrize(
    'database',
    ['postgres', 'mysql'],
    ids=["psql", "mysql"])
def tests(session, django, database):
    ...

Produces these sessions when running nox --list:

* tests(psql, old)
* tests(mysql, old)
* tests(psql, new)
* tests(mysql, new)

Parametrizing the session Python

You can use parametrization to select the Python interpreter for a session. These two examples are equivalent:

@nox.session
@nox.parametrize("python", ["3.6", "3.7", "3.8"])
def tests(session):
    ...

@nox.session(python=["3.6", "3.7", "3.8"])
def tests(session):
    ...

The first form can be useful if you need to exclude some combinations of Python versions with other parameters. For example, you may want to test against multiple versions of a dependency, but the latest version doesn't run on older Pythons:

@nox.session
@nox.parametrize(
    "python,dependency",
    [
        (python, dependency)
        for python in ("3.6", "3.7", "3.8")
        for dependency in ("1.0", "2.0")
        if (python, dependency) != ("3.6", "2.0")
    ],
)
def tests(session, dependency):
    ...

The session object

nox.sessions

Nox will call your session functions with a an instance of the Session class.

Session

Modifying Nox's behavior in the Noxfile

Nox has various command line arguments <usage> that can be used to modify its behavior. Some of these can also be specified in the Noxfile using nox.options. For example, if you wanted to store Nox's virtualenvs in a different directory without needing to pass it into nox every time:

import nox

nox.options.envdir = ".cache"

@nox.session
def tests(session):
    ...

Or, if you wanted to provide a set of sessions that are run by default:

import nox

nox.options.sessions = ["lint", "tests-3.6"]

...

The following options can be specified in the Noxfile:

  • nox.options.envdir is equivalent to specifying --envdir <opt-envdir>.
  • nox.options.sessions is equivalent to specifying -s or --sessions <opt-sessions-pythons-and-keywords>. If set to an empty list, no sessions will be run if no sessions were given on the commend line, and the list of available sessions will be shown instead.
  • nox.options.pythons is equivalent to specifying -p or --pythons <opt-sessions-pythons-and-keywords>.
  • nox.options.keywords is equivalent to specifying -k or --keywords <opt-sessions-pythons-and-keywords>.
  • nox.options.default_venv_backend is equivalent to specifying -db or --default-venv-backend <opt-default-venv-backend>.
  • nox.options.force_venv_backend is equivalent to specifying -fb or --force-venv-backend <opt-force-venv-backend>.
  • nox.options.reuse_existing_virtualenvs is equivalent to specifying --reuse-existing-virtualenvs <opt-reuse-existing-virtualenvs>. You can force this off by specifying --no-reuse-existing-virtualenvs during invocation.
  • nox.options.stop_on_first_error is equivalent to specifying --stop-on-first-error <opt-stop-on-first-error>. You can force this off by specifying --no-stop-on-first-error during invocation.
  • nox.options.error_on_missing_interpreters is equivalent to specifying --error-on-missing-interpreters <opt-error-on-missing-interpreters>. You can force this off by specifying --no-error-on-missing-interpreters during invocation.
  • nox.options.error_on_external_run is equivalent to specifying --error-on-external-run <opt-error-on-external-run>. You can force this off by specifying --no-error-on-external-run during invocation.
  • nox.options.report is equivalent to specifying --report <opt-report>.

When invoking nox, any options specified on the command line take precedence over the options specified in the Noxfile. If either --sessions or --keywords is specified on the command line, both options specified in the Noxfile will be ignored.

Nox version requirements

Nox version requirements can be specified in your Noxfile by setting nox.needs_version. If the Nox version does not satisfy the requirements, Nox exits with a friendly error message. For example:

import nox

nox.needs_version = ">=2019.5.30"

@nox.session(name="test")  # name argument was added in 2019.5.30
def pytest(session):
    session.run("pytest")

Any of the version specifiers defined in PEP 440 can be used.

Warning

Version requirements must be specified as a string literal, using a simple assignment to nox.needs_version at the module level. This allows Nox to check the version without importing the Noxfile.