Configuration & API


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

Defining sessions

session(func=None, python=None, py=None, reuse_venv=None, name=None, venv_backend=None)

Designate the decorated function as a session.

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

import nox

def tests(session):'pytest')

You can also configure sessions to run against multiple Python versions as described in Configuring a session’s virtualenv and parametrize sessions as described in parametrized sessions.

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

def tests(session):
    """Run the test suite."""'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

def a_very_long_function_name(session):

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"

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:

def tests(session):

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):

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):

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

@nox.session(python=['2.7', '3.5', '3.6', '3.7'])
def tests(session):

Will produce these sessions:

* tests-2.7
* tests-3.5
* tests-3.6
* tests-3.7

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:

def tests(session):

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

    python=['2.7', '3.6'],
def tests(session):

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:

def test(session):

    if session.posargs:
        test_files = session.posargs
        test_files = ['', '']'pytest', *test_files)

Now you if you run:


Then nox will run:


But if you run:

nox --

Then nox will run:


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

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

$ nox
nox > Running session tests(django='1.9')
nox > pip install django==1.9
nox > Running session tests(djano='2.0')
nox > pip install django==2.0

nox.parametrize() has an interface and usage intentionally similar to pytest’s parametrize.

parametrize(arg_names, arg_values_list, ids=None)

Parametrize a session.

Add new invocations to the underlying session function using the list of arg_values_list for the given arg_names. Parametrization is performed during session discovery and each invocation appears as a separate session to nox.

  • arg_names (Sequence[str]) – A list of argument names.
  • arg_values_list (Sequence[Union[Any, Tuple]]) – The list of argument values determines how often a session is invoked with different argument values. If only one argument name was specified then this is a simple list of values, for example [1, 2, 3]. If N argument names were specified, this must be a list of N-tuples, where each tuple-element specifies a value for its respective argument name, for example [(1, 'a'), (2, 'b')].
  • ids (Sequence[str]) – Optional sequence of test IDs to use for the parametrized arguments.

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

@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 Specifying parametrized 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. You can give parametrized sessions custom IDs to help in this scenario. These two examples are equivalent:

    ['1.9', '2.0'],
    ids=['old', 'new'])
def tests(session, django):
@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:

    ['1.9', '2.0'],
    ids=["old", "new"])
    ['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)

The session object

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

class Session(runner)

The Session object is passed into each user-defined session function.

This is your primary means for installing package and running commands in your Nox session.


The bin directory for the virtualenv.


An alias for chdir().


Change the current working directory.

conda_install(*args, **kwargs)

Install invokes conda install to install packages inside of the session’s environment.

To install packages directly:

session.conda_install('numpy', 'scipy')
session.conda_install('--channel=conda-forge', 'dask==2.1.0')

To install packages from a requirements.txt file:

session.conda_install('--file', 'requirements.txt')
session.conda_install('--file', 'requirements-dev.txt')

To install the current package without clobbering conda-installed dependencies:

session.install('.', '--no-deps')
# Install in editable mode.
session.install('-e', '.', '--no-deps')

Additional keyword args are the same as for run().


A dictionary of environment variables to pass into all commands.

error(*args, **kwargs)

Immediately aborts the session and optionally logs an error.

install(*args, **kwargs)

Install invokes pip to install packages inside of the session’s virtualenv.

To install packages directly:

session.install('requests', 'mock')

To install packages from a requirements.txt file:

session.install('-r', 'requirements.txt')
session.install('-r', 'requirements-dev.txt')

To install the current package:

# Install in editable mode.
session.install('-e', '.')

Additional keyword args are the same as for run().


Returns True if Nox is being run in an interactive session or False otherwise.

log(*args, **kwargs)

Outputs a log during the session.


Place the given session at the end of the queue.

This method is idempotent; multiple notifications to the same session have no effect.

Parameters:target (Union[str, Callable]) – The session to be notified. This may be specified as the appropriate string (same as used for nox -s) or using the function object.

This is set to any extra arguments passed to nox on the commandline.


The python version passed into @nox.session.

run(*args, env=None, **kwargs)

Run a command.

Commands must be specified as a list of strings, for example:'pytest', '-k', 'fast', 'tests/')'flake8', '--import-order-style=google')

You can not just pass everything as one string. For example, this will not work:'pytest -k fast tests/')

You can set environment variables for the command using env:
    'bash', '-c', 'echo $SOME_ENV',
    env={'SOME_ENV': 'Hello'})

You can also tell nox to treat non-zero exit codes as success using success_codes. For example, if you wanted to treat the pytest “tests discovered, but none selected” error as success:
    'pytest', '-k', 'not slow',
    success_codes=[0, 5])

On Windows, builtin commands like del cannot be directly invoked, but you can use cmd /c to invoke them:'cmd', '/c', 'del', 'docs/modules.rst')
  • env (dict or None) – A dictionary of environment variables to expose to the command. By default, all environment variables are passed.
  • silent (bool) – Silence command output, unless the command fails. False by default.
  • success_codes (list, tuple, or None) – A list of return codes that are considered successful. By default, only 0 is considered success.
  • external (bool) – If False (the default) then programs not in the virtualenv path will cause a warning. If True, no warning will be emitted. These warnings can be turned into errors using --error-on-external-run. This has no effect for sessions that do not have a virtualenv.
skip(*args, **kwargs)

Immediately skips the session and optionally logs a warning.


The virtualenv that all commands are run in.

Modifying Nox’s behavior in the Noxfile

Nox has various command line arguments 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"

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.
  • nox.options.sessions is equivalent to specifying -s or –sessions.
  • nox.options.keywords is equivalent to specifying -k or –keywords.
  • nox.options.reuse_existing_virtualenvs is equivalent to specifying –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. 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. 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. You can force this off by specifying --no-error-on-external-run during invocation.
  • is equivalent to specifying –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.