A mini-tutorial / working example / cheatsheet / link-collection to get you started documenting Python code using the Sphinx documentation system.

This was hastily put together for an Oxford Astrophysics code-coffee session in March 2017, so it's

• brief
• possibly erroneous
• perhaps out-of-date at time of reading

Treat accordingly.

Sphinx is a markup formatting engine / ecosystem, originally developed for documenting the Python programming language. You likely have already come across other markup formatters / formats such as LaTeX and MarkDown.

The format Sphinx uses is called 'reStructuredText', or 'RST' for short. It sits somewhere in-between MarkDown (simple, good for a single-page document with hyperlinks, maybe an image or two) and LaTeX (complex, good for a PhD thesis with huge tables and loads of images and cross-referenced equations and bibliographies). It's useful to understand the distinction - RST defines a basic formatting language which can be extended and customised, but isn't specifically aimed at Python.

Once you get past the basic formatting syntax of RST (e.g. bold, italic, code, hyperlink) you'll quickly come across the more powerful syntax elements, directives and roles. These are a way of defining a sort of custom formatting 'function'; a role is used for formatting or hyperlinking a short span of text, while a directive is used for formatting a whole block of text, and may have multiple options.

Sphinx adds a standard framework of useful extra roles and directives (and many optional extensions) that make it well suited to documenting Python code (and perhaps C++ if you're adventurous: see Breathe). Sphinx is designed primarily for creating HTML documentation, but can also create static PDFs etc.

We'll talk through the process of setting up Sphinx-docs from scratch in the coffee-session, but in case you need a reminder or are simply reading this online, check out the tutorials in the links below.

A set of Sphinx-docs is just a collection of RST files ('the source') and a Python file typically called conf.py which controls the configuration (HTML theme, Sphinx-extensions, etc etc). You build the source using the sphinx-build command, like so:

$ sphinx-build -b html sourcedir builddir

See sphinx-build --help for details.

If Sphinx can find your Python code (either because you've packaged it¹ and installed into into your working environment, or added it to sys.path in your conf.py - see tutorials), then you can use the autodoc extension to generate HTML pages from your in-code Python docstrings. Which suddenly become a lot more useful. (NB, make sure sphinx.ext.autodoc is in your extensions list in conf.py.)

If you're writing docstrings, you should probably be documenting your parameters and return types. I think the built-in format for this is ugly / hard-to-read, so I recommend the Sphinx-Napoleon extension.

If your code is openly-hosted on GitHub, you can use ReadTheDocs to host your docs with automatic updates (some configuration required).

• An RST 'cheat-sheet': https://github.com/ralsina/rst-cheatsheet/blob/master/rst-cheatsheet.rst
• The full RST language reference: http://docutils.sourceforge.net/docs/ref/rst/restructuredtext.html
• An online RST editor with live-preview (just remember to copy the text when done and save locally!): http://rst.ninjs.org/
• (Aside: there's a neat live-editor for MarkDown, too: http://dillinger.io/)

• Official tutorial: http://www.sphinx-doc.org/en/stable/tutorial.html
• Slightly more friendly unofficial tutorial: https://samnicholls.net/2016/06/15/how-to-sphinx-readthedocs/
• Sphinx-Napoleon, for nicer docstrings: http://sphinxcontrib-napoleon.readthedocs.io/en/latest/index.html
• Sphinx-Argparse, for documenting argparse driven command-line interfaces: https://sphinx-argparse.readthedocs.io/
• NBSphinx, for building documentation from Jupyter Notebooks: https://nbsphinx.readthedocs.io/
• ReadTheDocs, for auto-building and online hosting of your docs: https://docs.readthedocs.io/
• Fully-fledged example (of mine) including notebooks and more: http://voevent-parse.readthedocs.io/