Allow .nb.py as an alternative notebook extension

[mwouts]

Allowing companion python scripts to have .nb.py extension rather than .py would possibly improve the user experience, as it

• indicates more explicitly that the new file is a notebook
• would support the case when a python script exists with the same name than the notebook (but with another content)

[mwouts]

An initial implementation is available on branch v0.5.0. If I set the default config to

c.ContentsManager.default_nbrmd_formats = 'ipynb,nb.py'

then a jupyter notebook 'notebook.ipynb' is duplicated to a python only version with name 'notebook.nb.py', as expected. If I modify that file and reopen the notebook, then the inputs are up to date.

However, when I open a plain python file, say 'script.py' as a notebook, and save it, two additional files are created: 'script.ipynb' and also 'script.nb.py' (with contents identical to 'script.py'). Clearly

• the 'script.nb.py' is useless
• and the creation of 'script.ipynb' is questionable (see also nbrmd creates .py file although not specified.  jupytext#16), as it could override a pre-existing notebook with the same name.

I will at least avoid generating two notebooks with the same extensions (not generate 'script.nb.py'), and think a bit more to the second case: should 'ipynb' files be generated only for extensions that are in 'nbrmd_formats' ? Or even, should 'nbrmd_formats' become a dictionary that tells which formats to save to, given the opened one?

[mwouts]

The new format for nbrmd_formats, for version 0.5.0, is a list of extension groups. It is mostly compatible with the previous one.

• when nbrmd_formats=="ipynb" (the default value), then py and Rmd files can be opened as notebooks, and an ipynb file is created when they are saved. Outputs are taken from there when file is reloaded. However, opening the ipynb file does not read nor py nor Rmd file.
• when nbrmd_formats=="ipynb,py" then py is the source file for ipynb notebooks. Rmd notebook can be opened, but no ipynb file is created when Rmd is saved
• when nbrmd_formats=="ipynb,py;ipynb,Rmd" then py is the source file for ipynb notebooks. Rmd notebook can be opened, and a ipynb file is updated/created when the Rmd is saved
• when nbrmd_formats=="ipynb,py,Rmd" then py is the source file for ipynb notebooks. All three extensions are updated when one of these notebooks is saved.
• when nbrmd_formats=="ipynb,nb.py;script.ipynb,py" then nb.py is the source file for ipynb notebooks and script.ipynb is the output file for py files.

[grst]

This is indeed getting complicated.

I believe that instead of thinking of the configuration options first, we should think of the expected user experience first, and then how it can be implemented.

My expectations:

• when opening a file of a certain extension (.py, .nb.py, .Rmd, .rmd, ...) the file is saved with the same extension
• if configured correspondingly, create an .ipynb alongside to preserve outputs

To discuss

• When opening an .ipynb, either

  • save as .ipynb, no text-only format changed, or
  • save as .ipynb + a predefined text-only format

• When creating a new notebook, which extension to use?

  • ipynb only? -> in jupyterlab one can create arbitrary files manually
  • a text-only format specified in configuration (+ ipynb, if desired)

• Should we support saving multiple text-only formats for the same file, e.g. ipynb + Rmd + nb.py?

  • Personally, I don't see a use-case in having a synchronized copy of e.g. Rmd and .nb.py. I am therefor in favor of supporting only a single text-only format per notebook, as it keeps things simpler. There is still the CLI to convert to different formats, if required.