Error when using myst markdown notebooks and execute=="off"

[choldgraf]

If someone builds a Sphinx site with

• MyST-nb installed
• A MyST-markdown notebook
• Sets jupyter_execute = "off"

Then we trigger a bug somewhere in jupyter_sphinx when the parsing happens. Here seem to be the relevant lines from the traceback I found:

  File "/c/Users/choldgraf/Dropbox/github/forks/python/ebp/myst-nb/myst_nb/parser.py", line 72, in parse
    path_doc = nb_output_to_disc(ntbk, document)
  File "/c/Users/choldgraf/Dropbox/github/forks/python/ebp/myst-nb/myst_nb/parser.py", line 291, in nb_output_to_disc
    write_notebook_output(ntbk, str(output_dir), doc_filename)
  File "/home/choldgraf/anaconda/envs/dev/lib/python3.8/site-packages/jupyter_sphinx/execute.py", line 259, in write_notebook_output
    ext = notebook.metadata.language_info.file_extension
  File "/home/choldgraf/anaconda/envs/dev/lib/python3.8/site-packages/ipython_genutils/ipstruct.py", line 134, in __getattr__
    raise AttributeError(key)
AttributeError: language_info

I think that it might make sense to not support this use-case, since it is strange to have a myst markdown notebook that doesn't get executed. But, perhaps we want to raise a warning instead of having this error show up.

[chrisjsewell]

This is handled here when using the cache (and no cache match is found): https://github.com/ExecutableBookProject/MyST-NB/blob/b750c1aaefac83bf586da90809a485fa7fd09d30/myst_nb/cache.py#L185-L191

But yes it doesn't make much sense to be using the markdown notebooks with no execution or caching, so this should be warned or raised