Tutorial

This tutorial will walk you through installing, configuring, and running Nox.

Installation

Nox can be easily installed via pip:

python3 -m pip install nox

You may want to use the user site to avoid messing with your global Python install:

python3 -m pip install --user nox

Or you can be extra fancy and use pipx:

pipx install nox

Either way, Nox is usually installed globally, similar to tox, pip, and other similar tools.

If you’re interested in running nox within docker, you can use the thekevjames/nox images on DockerHub which contain builds for all nox versions and all supported python versions. Nox is also supported via pipx run nox in the manylinux images.

If you want to run nox within GitHub Actions, you can use the wntrblm/nox action, which installs the latest nox and makes available all active CPython and PyPY versions provided by the GitHub Actions environment:

# setup nox with all active CPython and PyPY versions provided by
# the GitHub Actions environment i.e.
# python-versions: "3.8, 3.9, 3.10, 3.11, 3.12, pypy-3.8, pypy-3.9, pypy-3.10"
# Any Nox tag will work here
- uses: wntrblm/nox@2024.04.15

# setup nox only for a given list of python versions
# Limitations:
# - Version specifiers shall be supported by actions/setup-python
# - There can only be one "major.minor" per interpreter i.e. "3.12.0, 3.12.1" is invalid
# Any Nox tag will work here
- uses: wntrblm/nox@2024.04.15
  with:
      python-versions: "2.7, 3.5, 3.11, pypy-3.9"

Writing the configuration file

Nox is configured via a file called noxfile.py in your project’s directory. This file is a Python file that defines a set of sessions. A session is an environment and a set of commands to run in that environment. If you’re familiar with tox sessions are analogous to environments. If you’re familiar with GNU Make, sessions are analogous to targets.

Sessions are declared using the @nox.session decorator. This is similar to how Flask uses @app.route.

Here’s a basic Noxfile that runs flake8 against example.py (you can create example.py yourself):

import nox

@nox.session
def lint(session):
    session.install("flake8")
    session.run("flake8", "example.py")

Running Nox for the first time

Now that you’ve installed Nox and have a Noxfile you can run Nox! Open your project’s directory in a terminal and run nox. You should see something like this:

$ nox
nox > Running session lint
nox > Creating virtualenv using python3.7 in .nox/lint
nox > pip install flake8
nox > flake8 example.py
nox > Session lint was successful.

✨ You’ve now successfully used Nox for the first time! ✨

The rest of this tutorial will take you through other common things you’ll likely want to do with Nox. You can also jump into Command-line usage and Configuration & API docs if you want.

Installing dependencies

Nox more or less passes session.install through to pip, so you can install stuff in the usual way. Here’s some examples:

To install one or more packages at a time:

@nox.session
def tests(session):
    # same as pip install pytest protobuf>3.0.0
    session.install("pytest", "protobuf>3.0.0")
    ...

To install a requirements.txt file:

@nox.session
def tests(session):
    # same as pip install -r requirements.txt
    session.install("-r", "requirements.txt")
    ...

If your project is a Python package and you want to install it:

@nox.session
def tests(session):
    # same as pip install .
    session.install(".")
    ...

In some cases such as Python binary extensions, your package may depend on code compiled outside of the Python ecosystem. To make sure a low-level dependency (e.g. libfoo) is available during installation:

@nox.session
def tests(session):
    ...
    session.run_install(
        "cmake", "-DCMAKE_BUILD_TYPE=Debug",
        "-S", libfoo_src_dir,
        "-B", build_dir,
        external=True,
    )
    session.run_install(
        "cmake",
        "--build", build_dir,
        "--config", "Debug",
        "--target", "install",
        external=True,
    )
    session.install(".")
    ...

These commands will run even if you are only installing, and will not run if the environment is being reused without reinstallation.

Loading dependencies from pyproject.toml or scripts

One common need is loading a dependency list from a pyproject.toml file (say to prepare an environment without installing the package) or supporting PEP 723 scripts. Nox provides a helper to load these with nox.project.load_toml. It can be passed a filepath to a toml file or to a script file following PEP 723. For example, if you have the following peps.py:

# /// script
# requires-python = ">=3.11"
# dependencies = [
#   "requests<3",
#   "rich",
# ]
# ///

import requests
from rich.pretty import pprint

resp = requests.get("https://peps.python.org/api/peps.json")
data = resp.json()
pprint([(k, v["title"]) for k, v in data.items()][:10])

You can make a session for it like this:

@nox.session
def peps(session):
    requirements = nox.project.load_toml("peps.py")["dependencies"]
    session.install(*requirements)
    session.run("peps.py")

Running commands

The session.run function lets you run commands within the context of your session’s virtual environment. Here’s a few examples:

You can install and run Python tools:

@nox.session
def tests(session):
    session.install("pytest")
    session.run("pytest")

If you want to pass more arguments to a program just add more arguments to run:

@nox.session
def tests(session):
    session.install("pytest")
    session.run("pytest", "-v", "tests")

You can also pass environment variables:

@nox.session
def tests(session):
    session.install("pytest")
    session.run(
        "pytest",
        env={
            "FLASK_DEBUG": "1"
        }
    )

See nox.sessions.Session.run() for more options and examples for running programs.

Selecting which sessions to run

Once you have multiple sessions in your Noxfile you’ll notice that Nox will run them all by default. While this is useful, it often useful to just run one or two at a time. You can use the --sessions argument (or -s) to select which sessions to run. You can use the --list argument to show which sessions are available and which will be run. Here’s some examples:

Here’s a Noxfile with three sessions:

import nox

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

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

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

If you just run nox --list you’ll see that all sessions are selected:

Sessions defined in noxfile.py:

* test
* lint
* docs

sessions marked with * are selected,
sessions marked with - are skipped.

If you run nox --list --sessions lint you’ll see that only the lint session is selected:

Sessions defined in noxfile.py:

- test
* lint
- docs

sessions marked with * are selected,
sessions marked with - are skipped.

And if you run nox --sessions lint Nox will just run the lint session:

nox > Running session lint
nox > Creating virtualenv using python3 in .nox/lint
nox > ...
nox > Session lint was successful.

In the Noxfile, you can specify a default set of sessions to run. If so, a plain nox call will only trigger certain sessions:

import nox

nox.options.sessions = ["lint", "test"]

If you set this to an empty list, Nox will not run any sessions by default, and will print a helpful message with the --list output when a user does not specify a session to run.

There are many more ways to select and run sessions! You can read more about invoking Nox in Command-line usage.

Queuing sessions

If you want to queue up (or “notify”) another session from the current one, you can use the session.notify function:

@nox.session
def tests(session):
    session.install("pytest")
    session.run("pytest")
    # Here we queue up the test coverage session to run next
    session.notify("coverage")

@nox.session
def coverage(session):
    session.install("coverage")
    session.run("coverage")

You can queue up any session you want, not just test and coverage sessions, but this is a very commonly used pattern.

Now running nox --session tests will run the tests session and then the coverage session.

You can also pass the notified session positional arguments:

@nox.session
def prepare_thing(session):
    thing_path = "./path/to/thing"
    session.run("prepare", "thing", thing_path)
    session.notify("consume_thing", posargs=[thing_path])

@nox.session
def consume_thing(session):
    # The 'consume' command has the arguments
    # sent to it from the 'prepare_thing' session
    session.run("consume", "thing", *session.posargs)

Note that this will only have the desired effect if selecting sessions to run via the --session/-s flag. If you simply run nox, all selected sessions will be run.

Testing against different and multiple Pythons

Many projects need to support either a specific version of Python or multiple Python versions. You can have Nox run your session against multiple interpreters by specifying python to @nox.session. Here’s some examples:

If you want your session to specifically run against a single version of Python only:

@nox.session(python="3.12")
def test(session):
    ...

If you want your session to run against multiple versions of Python:

@nox.session(python=["3.10", "3.11", "3.12"])
def test(session):
    ...

You’ll notice that running nox --list will show that this one session has been expanded into three distinct sessions:

Sessions defined in noxfile.py:

* test-3.10
* test-3.11
* test-3.12

You can run all of the test sessions using nox --sessions test or run an individual one using the full name as displayed in the list, for example, nox --sessions test-3.12. More details on selecting sessions can be found over in the Command-line usage documentation.

You can read more about configuring the virtual environment used by your sessions over at Configuring a session’s virtualenv.

Testing with conda

Some projects, especially in the data science community, need to test that they work in a conda environment. If you want your session to run in a conda environment:

@nox.session(venv_backend="conda")
def test(session):
    ...

Install packages with conda:

session.conda_install("pytest", channels=["conda-forge"])

It is possible to install packages with pip into the conda environment, but it’s a best practice only install pip packages with the --no-deps option. This prevents pip from breaking the conda environment by installing incompatible versions of packages already installed with conda. You should always specify channels for consistency; default channels can vary (and micromamba has none).

session.install("contexter", "--no-deps")
session.install("-e", ".", "--no-deps")

"mamba" is also allowed as a choice for venv_backend, which will use/require mamba instead of conda.

"micromamba" is also allowed as a choice for venv_backend, which will use/require micromamba instead of conda.

Parametrization

Just like Nox can handle running against multiple interpreters, Nox can also handle running your sessions with a list of different arguments using the nox.parametrize() decorator.

Here’s a short example of using parametrization to test against two different versions of Django:

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

If you run nox --list you’ll see that Nox expands your one session into multiple sessions. One for each argument value that you want to be passed to your session:

Sessions defined in noxfile.py:

* test(django='1.9')
* test(django='2.0')

nox.parametrize() has an interface and usage intentionally similar to pytest’s parametrize. It’s an extremely powerful feature of Nox. You can read more about parametrization and see more examples over at Parametrizing sessions.

Session tags

You can add tags to your sessions to help you organize your development tasks:

@nox.session(tags=["style", "fix"])
def black(session):
    session.install("black")
    session.run("black", "my_package")

@nox.session(tags=["style", "fix"])
def isort(session):
    session.install("isort")
    session.run("isort", "my_package")

@nox.session(tags=["style"])
def flake8(session):
    session.install("flake8")
    session.run("flake8", "my_package")

If you run nox -t style, Nox will run all three sessions:

* black
* isort
* flake8

If you run nox -t fix, Nox will only run the black and isort sessions:

* black
* isort
- flake8

If you run nox -t style fix, Nox will run all sessions that match any of the tags, so all three sessions:

* black
* isort
* flake8

Next steps

Look at you! You’re now basically an expert at Nox! ✨

For this point you can:

For any projects using Nox, you may add its official badge somewhere prominent like the README.

Nox 
[![Nox](https://img.shields.io/badge/%F0%9F%A6%8A-Nox-D85E00.svg)](https://github.com/wntrblm/nox)

Have fun! 💜