Prevent adding a source suffix twice

[martinRenou]

PR coupled with jupyterlite/jupyterlite-sphinx#28

This prevents collisions between nbsphinx and jupyterlite-sphinx which both add the ".ipynb" source prefix: if app.registry.source_suffix already contains "ipynb", then do nothing.

The behavior will then be the following:

conf.py

# nbsphinx will handle .ipynb files and jupyterlite-sphinx won't
extensions = [
    'nbsphinx',
    'jupyterlite_sphinx',
]

# jupyterlite-sphinx will handle .ipynb files and nbsphinx won't
extensions = [
    'jupyterlite_sphinx',
    'nbsphinx',
]

[martinRenou]

I can really see documentation taking advantages of both extensions.

[mgeier]

Thanks for this PR!

[mgeier]

I don't think that the nbsphinx_custom_formats have to be checked further. They are provided by the user, so it's the user's responsibility not to provide conflicting suffixes.

[martinRenou]

I see. Does this nbsphinx_custom_formats config do anything else that sphinx's source_suffix don't do? https://www.sphinx-doc.org/en/master/usage/configuration.html#confval-source_suffix

so it's the user's responsibility not to provide conflicting suffixes

Actually my understanding is that sphinx's source_suffix is a way for users to overwrite anything that extensions have setup, so it's not really the same thing? Or am I completely misunderstanding all this? (sphinx beginner here)

[mgeier]

Does this nbsphinx_custom_formats config do anything else that sphinx's source_suffix don't do?

Yes. In addition to registering the source suffix, it also provides a function that transforms a string containing certain custom format into a Jupyter notebook object. See https://nbsphinx.readthedocs.io/custom-formats.html.

But I assume that if a user provides a custom format, they don't want their chosen suffix to be overwritten by some other Sphinx extension.

And if another extension is loaded after nbsphinx, I prefer an error over silently ignoring the source suffix.

And if a user provides nbsphinx_custom_formats and a conflicting source_suffix, it's their own fault.

[mgeier]

@martinRenou Do you want to update this PR or should I do it?

[martinRenou]

Please do it if you can :) Thanks a lot!

[mgeier]

I have rebased your branch and I've added fc79fd1 that unconditionally adds nbsphinx_custom_formats.

Can you please test if that works for you?

[martinRenou]

Thanks a lot! I'll give it a try

[mgeier]

I've merged this PR already, but please let me know if it doesn't work as expected, so we can fix that.