init_printing/init_session shouldn't generate png math by default

[asmeurer]

If you call init_printing or init_session in a Jupyter notebook, by default it generates both mathjax and png output. For example, this notebook

generates this JSON:

{
 "cells": [
  {
   "cell_type": "code",
   "execution_count": 1,
   "id": "a724c220",
   "metadata": {},
   "outputs": [],
   "source": [
    "from sympy import *\n",
    "init_printing()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "id": "4d700e39",
   "metadata": {},
   "outputs": [
    {
     "data": {
      "image/png": "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\n",
      "text/latex": [
       "$\\displaystyle \\frac{1}{\\pi}$"
      ],
      "text/plain": [
       "1\n",
       "─\n",
       "π"
      ]
     },
     "execution_count": 3,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "1/pi"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "id": "2791a4c8",
   "metadata": {},
   "outputs": [],
   "source": []
  }
 ],
 "metadata": {
  "kernelspec": {
   "display_name": "Python 3 (ipykernel)",
   "language": "python",
   "name": "python3"
  },
  "language_info": {
   "codemirror_mode": {
    "name": "ipython",
    "version": 3
   },
   "file_extension": ".py",
   "mimetype": "text/x-python",
   "name": "python",
   "nbconvert_exporter": "python",
   "pygments_lexer": "ipython3",
   "version": "3.9.15"
  }
 },
 "nbformat": 4,
 "nbformat_minor": 5
}

Notice the "image/png": "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\n",, which is a png of the LaTeX:

It does this by default as long as latex is installed.

This can be controlled by the use_latex flag. If it is True (the default), it generates both png and mathjax.

However, I don't know if it really should generate a png by default.

• It is slow (it calls out to latex to generate the png with each output)
• It makes the notebook file size much bigger than it otherwise would be
• PNGs are generally bad for accessibility and look worse than natively rendered latex with something like mathjax.
• Some notebook renderers will prefer them over latex, resulting in worse looking output. This is an upstream issue with jupyter, but it's been an issue forever.

I think we should change the default of use_latex to be 'mathjax'. The only benefit of png images is in renderers that don't support mathjax, like the qtconsole. We can detect if the frontend is the notebook and default to mathjax there and the current True otherwise.

[moorepants]

I use the suggested default in my "Learn Multibody Dynamics" text: me.init_vprinting(use_latex='mathjax') because jupyter-sphinx will inject the pngs instead of mathjax for sympy output if you don't set it. See issue moorepants/learn-multibody-dynamics#95

One possible downside, is that when you aren't connected to the internet mathjax may not work. I'm not sure what happens when use_latex is mathjax and there is no internet.

[asmeurer]

I just tested it and when I turn my internet off, the notebook can still render mathjax, because mathjax is installed locally in one of the jupyter packages.

However, I did try converting the notebook to html using nbconvert, and that does load mathjax from the CDN. If I turn my internet off, it just shows the raw latex. It doesn't seem to fallback to png, and as far as I can tell, the pngs aren't even saved there.

[moorepants]

I've noticed (seemingly) that mathjax can sometimes be left in the browser cache and it works without internet but then sometimes it doesn't. But I'm mostly working with jupyter-sphinx which likely uses nbconvert under the hood.

[asmeurer]

This was prompted by this issue jupyter-book/jupyter-book#1833. I don't know what the relationship is between jupyter-sphinx and jupyter-book, but with jupyter-book, notebooks with pngs saved in them will render the png instead of the latex.

[moorepants]

It is possible the jupyter-book uses jupyter-sphinx. I'm not that familiar with jupyter-book though. Either way, they do similar things.

[moorepants]

Looks like jupyter-book used to depend on jupyter-sphinx: https://github.com/search?q=org%3Aexecutablebooks+jupyter-sphinx&type=code

[asmeurer]

I have a fix at #25374. I thought this might break the qtconsole but it apparently still uses png even with this change.