Size reduction aproaches

[agriyakhetarpal]

Currently, we generate notebooks for the TryExamples directive, and the JupyterLite, NotebookLite, and Voici directives can take a path to a notebook.

While we can disable the JupyterLite source maps to reduce build size (a technique used downstream in scikit-learn/scikit-learn#26246, numpy/numpy#26745, and sympy/sympy#27419), I've opened this issue as an open-ended item to see if jupyterlite-sphinx as a companion to JupyterLite can reduce its footprint by reducing the size of the notebooks that are eventually copied into the JupyterLite folder.

• Use https://github.com/arve0/ipynbcompress? It has not been maintained for the last two years; maybe we can fork it or take over maintenance via PEP 541?
• Use optipng (if available) or projects with Python bindings to it, or pillow as an optional dependencies to reduce the size of images in the docs: https://sphinx-gallery.github.io/stable/gen_modules/sphinx_gallery.utils.optipng.html
• Use nbstripout if enabled via a global config option to clear all outputs (except the jupyterlite_sphinx_strip tag) and kernel metadata from all notebooks (maybe not for the TryExamples notebooks, but this would be useful for long-form notebooks – the scikit-learn docs via Sphinx-Gallery already to seem to do this: https://sphinx-gallery.github.io/stable/auto_examples/plot_9_multi_image_separate.html.
  • We don't currently do this for Markdown notebooks that were added in Support the usage of Markdown-based notebooks with "Lite" directives #221. Since they don't contain the outputs of the cells in their contents, they don't have any outputs upon conversion to IPyNB either.
  • However, conventional IPyNB files can indeed contain outputs, which we can explore stripping.

Reductions in sizes will be helpful for:

• projects deploying documentation via GitHub Pages or on other static webpage hosts
• reducing bandwidth usage for readers of said documentation

[agriyakhetarpal]

Okay, so, as an experiment, I tried reducing the size of the notebooks generated by the TryExamples directive – thinking that those notebooks have more significant numbers in comparison to the ones connected to the NotebookLite/JupyterLite directive(s) since they can quickly go into the thousands, based on how many docstrings exist in the entire documentation source for a package – sadly the results are not helpful. :(

By removing the outputs from the UUID-based notebooks from numpy/numpy#26745, the size of 1496 total notebooks was reduced from 4.5 MiB to 2.5 MiB, and a similar test for SymPy with 2519 notebooks/example revealed a reduction from 6.8 MiB to 3.9 MiB. Hence, this sounds like a paltry improvement of just 1.44% and is not really worth incorporating, especially when NumPy's total docs size without JupyterLite's source maps is ~138 MiB. Even with enabling global docstring examples for Matplotlib, which also uses Sphinx-Gallery and has a lot of images in its notebook outputs, I didn't see much of a reduction (20 MiB – brought down to 557 MiB).

I'll leave this issue open in case there's something I am missing in this aim to reduce the build size from jupyterlite-sphinx's side that anyone else can point out. Otherwise, we can close and try to find optimisation options in JupyterLite itself.