Describing code in Sphinx
=========================

In the :doc:`previous sections of the tutorial </tutorial/index>` you can read
how to write narrative or prose documentation in Sphinx. In this section you
will describe code objects instead.

Sphinx supports documenting code objects in several languages, namely Python,
C, C++, JavaScript, and reStructuredText. Each of them can be documented using
a series of directives and roles grouped by
:doc:`domain </usage/domains/index>`. For the remainder of the
tutorial you will use the Python domain, but all the concepts seen in this
section apply for the other domains as well.

.. _tutorial-describing-objects:

Python
------

Documenting Python objects
~~~~~~~~~~~~~~~~~~~~~~~~~~

Sphinx offers several roles and directives to document Python objects,
all grouped together in :doc:`the Python domain </usage/domains/python>`.
For example, you can use the :rst:dir:`py:function` directive to document
a Python function, as follows:

.. code-block:: rst
   :caption: docs/source/usage.rst

   Creating recipes
   ----------------

   To retrieve a list of random ingredients,
   you can use the ``lumache.get_random_ingredients()`` function:

   .. py:function:: lumache.get_random_ingredients(kind=None)

      Return a list of random ingredients as strings.

      :param kind: Optional "kind" of ingredients.
      :type kind: list[str] or None
      :return: The ingredients list.
      :rtype: list[str]

Which will render like this:

.. figure:: /_static/tutorial/lumache-py-function.png
   :width: 80%
   :align: center
   :alt: HTML result of documenting a Python function in Sphinx

   The rendered result of documenting a Python function in Sphinx

Notice several things:

- Sphinx parsed the argument of the ``.. py:function`` directive and
  highlighted the module, the function name, and the parameters appropriately.
- The directive content includes a one-line description of the function,
  as well as an :ref:`info field list <info-field-lists>` containing the function
  parameter, its expected type, the return value, and the return type.

.. note::

   The ``py:`` prefix specifies the :term:`domain`. You may configure the
   default domain so you can omit the prefix, either globally using the
   :confval:`primary_domain` configuration, or use the
   :rst:dir:`default-domain` directive to change it from the point it is called
   until the end of the file.
   For example, if you set it to ``py`` (the default), you can write
   ``.. function::`` directly.

Cross-referencing Python objects
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

By default, most of these directives generate entities that can be
cross-referenced from any part of the documentation by using
:ref:`a corresponding role <python-xref-roles>`. For the case of functions,
you can use :rst:role:`py:func` for that, as follows:

.. code-block:: rst
   :caption: docs/source/usage.rst

   The ``kind`` parameter should be either ``"meat"``, ``"fish"``,
   or ``"veggies"``. Otherwise, :py:func:`lumache.get_random_ingredients`
   will raise an exception.

When generating code documentation, Sphinx will generate a
cross-reference automatically just by using the name of the object,
without you having to explicitly use a role for that. For example, you
can describe the custom exception raised by the function using the
:rst:dir:`py:exception` directive:

.. code-block:: rst
   :caption: docs/source/usage.rst

   .. py:exception:: lumache.InvalidKindError

      Raised if the kind is invalid.

Then, add this exception to the original description of the function:

.. code-block:: rst
   :caption: docs/source/usage.rst
   :emphasize-lines: 7

   .. py:function:: lumache.get_random_ingredients(kind=None)

      Return a list of random ingredients as strings.

      :param kind: Optional "kind" of ingredients.
      :type kind: list[str] or None
      :raise lumache.InvalidKindError: If the kind is invalid.
      :return: The ingredients list.
      :rtype: list[str]

And finally, this is how the result would look:

.. figure:: /_static/tutorial/lumache-py-function-full.png
   :width: 80%
   :align: center
   :alt: HTML result of documenting a Python function in Sphinx
         with cross-references

   HTML result of documenting a Python function in Sphinx with cross-references

Beautiful, isn't it?

Including doctests in your documentation
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Since you are now describing code from a Python library, it will become useful
to keep both the documentation and the code as synchronized as possible.
One of the ways to do that in Sphinx is to include code snippets in the
documentation, called *doctests*, that are executed when the documentation is
built.

To demonstrate doctests and other Sphinx features covered in this tutorial,
Sphinx will need to be able to import the code. To achieve that, write this
at the beginning of ``conf.py``:

.. code-block:: python
   :caption: docs/source/conf.py
   :emphasize-lines: 3-5

   # If extensions (or modules to document with autodoc) are in another directory,
   # add these directories to sys.path here.
   import pathlib
   import sys
   sys.path.insert(0, pathlib.Path(__file__).parents[2].resolve().as_posix())

.. note::

   An alternative to changing the :py:data:`sys.path` variable is to create a
   ``pyproject.toml`` file and make the code installable,
   so it behaves like any other Python library. However, the ``sys.path``
   approach is simpler.

Then, before adding doctests to your documentation, enable the
:doc:`doctest </usage/extensions/doctest>` extension in ``conf.py``:

.. code-block:: python
   :caption: docs/source/conf.py
   :emphasize-lines: 3

   extensions = [
       'sphinx.ext.duration',
       'sphinx.ext.doctest',
   ]

Next, write a doctest block as follows:

.. code-block:: rst
   :caption: docs/source/usage.rst

   >>> import lumache
   >>> lumache.get_random_ingredients()
   ['shells', 'gorgonzola', 'parsley']

Doctests include the Python instructions to be run preceded by ``>>>``,
the standard Python interpreter prompt, as well as the expected output
of each instruction. This way, Sphinx can check whether the actual output
matches the expected one.

To observe how a doctest failure looks like (rather than a code error as
above), let's write the return value incorrectly first. Therefore, add a
function ``get_random_ingredients`` like this:

.. code-block:: python
   :caption: lumache.py

   def get_random_ingredients(kind=None):
       return ["eggs", "bacon", "spam"]

You can now run ``make doctest`` to execute the doctests of your documentation.
Initially this will display an error, since the actual code does not behave
as specified:

.. code-block:: console

   (.venv) $ make doctest
   Running Sphinx v4.2.0
   loading pickled environment... done
   ...
   running tests...

   Document: usage
   ---------------
   **********************************************************************
   File "usage.rst", line 44, in default
   Failed example:
       lumache.get_random_ingredients()
   Expected:
       ['shells', 'gorgonzola', 'parsley']
   Got:
       ['eggs', 'bacon', 'spam']
   **********************************************************************
   ...
   make: *** [Makefile:20: doctest] Error 1

As you can see, doctest reports the expected and the actual results,
for easy examination. It is now time to fix the function:

.. code-block:: python
   :caption: lumache.py
   :emphasize-lines: 2

   def get_random_ingredients(kind=None):
       return ["shells", "gorgonzola", "parsley"]

And finally, ``make doctest`` reports success!

For big projects though, this manual approach can become a bit tedious.
In the next section, you will see :doc:`how to automate the
process </tutorial/automatic-doc-generation>`.

Other languages (C, C++, others)
--------------------------------

Documenting and cross-referencing objects
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Sphinx also supports documenting and cross-referencing objects written in
other programming languages. There are four additional built-in domains:
C, C++, JavaScript, and reStructuredText. Third-party extensions may
define domains for more languages, such as

- `Fortran <https://sphinx-fortran.readthedocs.io>`_,
- `Julia <https://bastikr.github.io/sphinx-julia>`_, or
- `PHP <https://github.com/markstory/sphinxcontrib-phpdomain>`_.

For example, to document a C++ type definition, you would use the built-in
:rst:dir:`cpp:type` directive, as follows:

.. code-block:: rst

   .. cpp:type:: std::vector<int> CustomList

      A typedef-like declaration of a type.

Which would give the following result:

.. cpp:type:: std::vector<int> CustomList

   A typedef-like declaration of a type.

All such directives then generate references that can be
cross-referenced by using the corresponding role. For example, to reference
the previous type definition, you can use the :rst:role:`cpp:type` role
as follows:

.. code-block:: rst

   Cross reference to :cpp:type:`CustomList`.

Which would produce a hyperlink to the previous definition: :cpp:type:`CustomList`.
