Tutorial
========

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


Installation
------------

Nox can be easily installed via `pip`_:

.. code-block:: console

    python3 -m pip install nox

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

.. code-block:: console

    python3 -m pip install --user nox

Or you can be extra fancy and use `pipx`_:

.. code-block:: console

    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:

.. code-block:: yaml

    # 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"

.. _pip: https://pip.readthedocs.org
.. _user site: https://packaging.python.org/tutorials/installing-packages/#installing-to-the-user-site
.. _pipx: https://packaging.python.org/guides/installing-stand-alone-command-line-tools/
.. _docker: https://www.docker.com/
.. _thekevjames/nox images: https://hub.docker.com/r/thekevjames/nox
.. _GitHub Actions: https://github.com/features/actions
.. _manylinux images: https://github.com/pypa/manylinux

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")

.. _flake8: http://flake8.pycqa.org/en/latest/


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:

.. code-block:: console

    $ 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 :doc:`usage` and
:doc:`config` 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:

.. code-block:: python

    @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:

.. code-block:: python

    @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:

.. code-block:: python

    @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:

.. code-block:: python

    @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 <https://peps.python.org/pep-0723>`_ 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``:


.. code-block:: python

    # /// 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:

.. code-block:: python

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

This is a common structure for scripts following this PEP, so a helper for it
is provided:

.. code-block:: python

   @nox.session
   def peps(session):
       session.install_and_run_script("peps.py")


Other helpers for ``pyproject.toml`` based projects are also available in
``nox.project``.

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:

.. code-block:: python

    @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``:

.. code-block:: python

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


You can also pass environment variables:

.. code-block:: python

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

See :func:`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:

.. code-block:: python

    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:

.. code-block:: console

    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:

.. code-block:: console

    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:

.. code-block:: console

    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:

.. code-block:: python

    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 :doc:`usage`.

Queuing sessions
-----------------

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

.. code-block:: python

    @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:

.. code-block:: python

    @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.

Requiring sessions
------------------

You can also request sessions be run before your session runs. This is done with the ``requires=`` keyword:


.. code-block:: python

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

    @nox.session(requires=["tests"])
    def coverage(session):
        session.install("coverage")
        session.run("coverage")

The required sessions will be stably topologically sorted and run. Parametrized
sessions are supported. You can also get the current Python version with
``{python}``, though arbitrary parametrizations are not supported.


.. code-block:: python

    @nox.session(python=["3.10", "3.13"])
    def tests(session):
        session.install("pytest")
        session.run("pytest")

    @nox.session(python=["3.10", "3.13"], requires=["tests-{python}"])
    def coverage(session):
        session.install("coverage")
        session.run("coverage")

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:

.. code-block:: python

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

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

.. code-block:: 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:

.. code-block:: console

    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 :doc:`usage` documentation.

You can read more about configuring the virtual environment used by your
sessions over at :ref:`virtualenv config`.


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:

.. code-block:: python

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

Install packages with conda:

.. code-block:: python

    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).

.. code-block:: python

    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 <https://github.com/mamba-org/mamba>`_ instead of conda.

``"micromamba"`` is also allowed as a choice for ``venv_backend``, which will
use/require `micromamba <https://mamba.readthedocs.io/en/latest/user_guide/micromamba.html#>`_
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
:func:`nox.parametrize` decorator.

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

.. code-block:: python

    @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:

.. code-block:: console

    Sessions defined in noxfile.py:

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


:func:`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
:ref:`parametrized`.

.. _pytest's parametrize: https://pytest.org/latest/parametrize.html#_pytest.python.Metafunc.parametrize


.. _session tags:

Session tags
------------

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

.. code-block:: python

    @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:

.. code-block:: console

    * black
    * isort
    * flake8


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

.. code-block:: console

    * 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:

.. code-block:: console

    * black
    * isort
    * flake8


Running without the nox command or adding dependencies
------------------------------------------------------

With a few small additions to your noxfile, you can support running using only
a generalized Python runner, such as ``pipx run noxfile.py``, ``uv run
noxfile.py``, ``pdm run noxfile.py``, or ``hatch run noxfile.py``. You need to
have the following comment in your noxfile:

.. code-block:: python

   # /// script
   # dependencies = ["nox"]
   # ///

And the following block of code:

.. code-block:: python

   if __name__ == "__main__":
       nox.main()

If this comment block is present, nox will also read it, and run a custom
environment (``_nox_script_mode``) if the dependencies are not met in the
current environment. This allows you to specify dependencies for your noxfile
or a minimum version of nox here (``requires-python`` version setting not
supported yet, but planned). You can control this with
``--script-mode``/``NOX_SCRIPT_MODE``; ``none`` will deactivate it, and
``fresh`` will rebuild it; the default is ``reuse``. You can also set
``--script-venv-backend``/``tool.nox.script-venv-backend``/``NOX_SCRIPT_VENV_BACKEND``
to control the backend used; the default is ``"uv|virtualenv"``.

Next steps
----------

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

For this point you can:

* Read more docs, such as :doc:`usage` and :doc:`config`.
* Give us feedback or contribute, see :doc:`CONTRIBUTING`.

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

.. tabs::

   .. tab:: Markdown

      .. code-block:: markdown

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

   .. tab:: reStructuredText

        .. code-block:: rst

            .. image:: https://img.shields.io/badge/%F0%9F%A6%8A-Nox-D85E00.svg
               :alt: Nox
               :target: https://github.com/wntrblm/nox

Have fun! 💜
