WIP - Filter empty cells

[martinRenou]

No description provided.

[vidartf]

As long as the filter does not remove any cell cells where the output is populated later (I'm not sure what cell.outputs is in that case), this is OK as well.

[maartenbreddels]

My preference goes to solving this in the template, and for instance not do this in the debug template.

[martinRenou]

Well, we actually still do that in the template. The only difference is that we don't add empty cells in the page, instead of only making them invisible.

[vidartf]

A way to get an empty output (but I guess this makes the cell be not empty, so this should still be ok):

display({}, raw=True, display_id=True)

[martinRenou]

But:

handle = display({}, raw=True, display_id=True)
handle.update({'text/html': 'test'})

does not seem to show anything (in classic Notebook)

[vidartf]

It works in lab though.

[vidartf]

Xref ipython/ipython#11907

[martinRenou]

It works in lab though.

And actually it works in Voila as well. Both using master and this branch. The output appears empty but it's not.

'outputs': [{'output_type': 'display_data', 'metadata': {}, 'data': {}}]

[vidartf]

Yes, but the question is then: Does #391 work?

[martinRenou]

Yes, for the same reason. cell.outputs is not None in the template, so we don't hide it in CSS

[maartenbreddels]

As discussed, cells should always be returned, since voila has the option to show the source code. Also, filtering out cells is impossible, since we don't know a priori if a cell will not contain output, and to make the progress indicator work, we need to return as many cells as there are in the notebook.

I think that only leaves the option open for a template to have custom behavior for empty output cells, e.g. only have an if statement in the template:

{% if 'outputs' in cell and cell.outputs %>

[martinRenou]

Closing in favor of #391