Add option to Ignore certain cells metadata

[erdnaavlis]

I like to use NBExtensions Execute Time extension.

When using jupytext, a cell will be converted to .py as:

# + {"ExecuteTime": {"start_time": "2018-10-17T10:31:56.157308Z", "end_time": "2018-10-17T10:31:56.160823Z"}}
print("I'm a cell!")

It can be seen that it has a ExecuteTime metadata.

Would it be possible for the jupytext user to specify which metadata to keep track of and which to ignore?
In my case I don't need this info in the .py and, in fact, I don't want it because it adds noise to git diff...
But maybe other users would like to keep it in the scripts.

Please let me know your thoughts.

@mwouts comment from the discussion started on #101 :

As for your comment on metadata, I like very much the idea of filtering cell metadata. As a start we could skip the 'ExecuteTime' cell metadata for everyone (there are already a few cell metadata that jupytext does not include, cf. https://github.com/mwouts/jupytext/blob/master/jupytext/cell_metadata.py#L32). Unless you think it is important that for every notebook, one can tell which cell metadata to include/exclude? Please let me know your preference.

Regarding your comment @mwouts , I don't see a need to specify which metadata to ignore on a per notebook basis.

• (most effort aproach) Ideally, jupytext would have some sort of config option (maybe on the jupyter config file that it already uses currently?). And the user could specify there which metadatas to exclude besides the default ones on https://github.com/mwouts/jupytext/blob/master/jupytext/cell_metadata.py#L32
• (less effort aproach) just add the ExecuteTime to the list on https://github.com/mwouts/jupytext/blob/master/jupytext/cell_metadata.py#L32. Personally I don't see any use for this metadata on the scripts. But maybe other users will?

[mwouts]

Thanks @andrethrill . I like both the most effort and the less effort approach! In the present case I also think that ExecuteTime could always be filtered (note that, from v0.8.2 on, the cell metadata that are absent from the text representation are still preserved in the .ipynb file).

Maybe we should even ask the opposite question: which cell metadata are relevant in the text representation? For people using R Markdown, I think the relevant metadata is the one they input manually: figure size, should code or outputs be included, etc... From your previous example at #101 , the run_control is another input that should be preserved. However I see the editable, deletable flags, are you aware where they are coming from? Do you personally use them in the text file?

Finally, let's think to how the user could configure his preferences for metadata (see also #105)...

1. jupytext section in the header could have an optional entry like cell_metadata. No entry would mean default (to be defined, see above). Value true would mean keep all metadata. Value false would mean keep no metadata. And a comma separated list could be used to preserve a few metadata keys, in addition to the default. If the list contains a minus sign, then keys after the minus sign are not represented in the text notebook (is that too complex?)
2. I think it is important that this information belongs to the notebook itself, because when you share the notebook, the person that receives it may not have the same configuration
3. Still, a default configuration is comfortable! Say that the contents manager has a cell_metadata configuration, and the notebook has no explicit config. Then the CM's configuration could go to the notebook metadata.

[erdnaavlis]

Thanks @mwouts !

Maybe we should even ask the opposite question: which cell metadata are relevant in the text representation?

This is a good point. Personally, as I get more familiar with Jupytext the way I see it being more useful in my workflow would be to ideally stop versioning control the .ipnyb's and only git track the respective .pys (notebooks are a mess for version controlling....). Of course, this would require the notebooks to be fully recoverable from the respective scripts.

Here, 'fully' means up to the metadata that the user would choose to be of interest. Hence my point of giving such config option to the user.

From your previous example at #101 , the run_control is another input that should be preserved. However I see the editable, deletable flags, are you aware where they are coming from? Do you personally use them in the text file?

The editable and deletable flags are used by the same Frozen Cells extension I referred. It has an intermediate state between frozen and unfrozen where you can simply lock the cell. It's not editable, it's not deletable but it executes. I think it's important metadata to be kept.

1. jupytext section in the header could have an optional entry like cell_metadata. No entry would mean default (to be defined, see above). Value true would mean keep all metadata. Value false would mean keep no metadata. And a comma separated list could be used to preserve a few metadata keys, in addition to the default. If the list contains a minus sign, then keys after the minus sign are not represented in the text notebook (is that too complex?)

That seems reasonable to me. But see my comment below:

2. I think it is important that this information belongs to the notebook itself, because when you share the notebook, the person that receives it may not have the same configuration.

Good point! I didn't think about that. Still, I feel it to be a bit cumbersome to have to specify these details for every new notebook the user creates. I believe that each user may have different configs, but each user will very likely adopt the same configs across most of his/her notebooks. What do you think about this instead:

• Jupytext has global config option.
• The header entry you mentioned in 1. get's automatically populated by jupytext using the info in the global config option

This way the user has the option to personalize (add or remove specific metadata, without the - or + signs) it on a per-notebook basis. But, by default, all user notebooks assume the config the user previously defined globally.