Bundle extensions with your Sphinx theme#

Sphinx is great because it has a ton of useful extensions that let you grow its functionality. However, a downside of this is that users have to actually learn about those extensions and activate them manually. It’s not hard, but it’s a non-trivial amount of discovery work.

One way to solve this is for themes to bundle extensions on their own. This way they can include functionality via an extension rather than writing custom code on their own.

However, this doesn’t often happen, I think because it can be pretty confusing how to do so. I believe I’ve figured out the major gotchas to avoid and patterns to follow, so here’s a quick overview.

Use app.setup_extension to add the extension#

First off, the Sphinx docs mention app.setup_extension as “the” way to set up a Sphinx extension.

This activates the extension as if you’d put it in the extensions variable. However, it leaves out a lot of nuance.

Don’t forget the theme is activated after config-inited#

Many extensions add functionality in Sphinx events. These are emitted throughout the build process and you can hook into them with functions to modify the document etc.

However, your theme is activated after the config-inited event. That means that if you manually activate an extension, but that extension registers a callback that waits for config-inited, then nothing will happen.

So in general, if something doesn’t seem like it’s happening, then double check that your extension isn’t using a config-inited event hook.

Sphinx extension authors: don’t use config-inited!

Because of this limitation, I’d recommend that Sphinx extension authors use builder-inited rather than config-inited. It is also early in the build process, but after the theme is activated.

Workaround: You can try emitting your own config-inited#

I think I figured out a workaround to the above limitation in case you can’t control which events your extensions hook into. It’s based on the idea that Sphinx has a registry of event listeners at app.events.listeners. This is a dictionary of event-name:[list-of-listener-objects] pairs. When app.emit(event) is triggered, it loops through the respective list and runs each listener object.

So we should be able to just remove that list, then activate our extensions (which will add to the list), and then manually trigger the config-inited event. Try the following steps:

  1. Store a copy of the old event listeners. They’ve already been run (since config-inited has happened already), but we’ll keep them just in case.

    old_listeners = app.events.listeners["config-inited"]
  2. Replace the old listeners with an empty list. This means there are effectively no listeners for the event.

    app.events.listeners["config-inited"] = []
  3. Activate your extra extensions. You can now loop through the list of extensions you want to bundle and activate each one.

    for ext in your_extensions:

    This will add event listeners to config-inited for any extensions that require it. Any extension that has already been activated will be a no-op.

  4. Emit your own config-inited event. Now that you’ve activated the extensions and added their event listeners, re-emit the config-inited event. This will now only call the new listeners you’ve added.

    app.emit("config-inited", app.config)
  5. Combine the two lists of listeners. Finally, combine the two lists of listeners just so that your Sphinx application’s state matches what has actually happened (I have no idea if this is actually needed).

    # This prepends the list
    # ref: https://stackoverflow.com/questions/19736058/can-i-extend-list-in-python-with-prepend-elements-instead-of-append
    app.events.listeners["config-inited"][:0] = old_listeners 

Now you can let the Sphinx build move forward as normal. You will have manually emitted the config-inited event for your new extensions, and they’ll be picked up for any future events that happen.

Addendum: you could also make your theme an extension#

If you really don’t want to do the hacky steps above, another option is to ask users to add your theme as an extension as well as a theme. For example:

extensions = ["my_theme"]
html_theme = "my_theme"

However, I’ve found that this usually confuses people and is pretty clunky from a UX perspective. I’d much rather try to handle the complexity under the hood.