Passing custom resources to voila configuration

[mkcor]

Hello,

This PR overwrites #298 (previously submitted). Example use:

voila reveal.ipynb --template=reveal --reveal_transition=fade

The config additions are almost copied-pasted from https://github.com/jupyter/nbconvert/blob/76061f8df164a742fc039b2574508bd8cf7d96a1/nbconvert/exporters/slides.py#L105-L130 -- should I add a comment about it?

/cc @maartenbreddels

[SylvainCorlay]

I like the idea of allowing to specify resources through the VoilaConfiguration object. An issue here is that this adds reveal-specific configurables to voila-core, which we cannot do for all possible templates.

Maybe a first way to do this would have the VoilaConfiguration have a resources Dict trait, which would not be reveal-specific. This may make setting options with the command line a bit difficult though.

[mkcor]

@SylvainCorlay we are on the same wavelength... It's cumbersome to write JSON at the command line, but maybe great docs could make up for this inconvenience (I guess users will end up copying-pasting...).

Ok, so I'll implement the Dict trait.

[SylvainCorlay]

I wonder if we could hack traitlets to have a Bunch syntax in the command line so that we can do

voila --foo.bar=baz

for any valid name bar. But I don't think that it would be in the scope of this PR to go beyond Dict.

[mkcor]

Oh, I hadn't seen your comment about Bunch (which I hadn't heard of before). Please see 4144ed6.

I can see three additional improvements which would belong in this PR.

• Modularization (abstraction): replace this if block by a function call, say resources.dict_merge(extra_resources) along the lines of these suggestions; is it a good idea to start a utils.py submodule in voila?
• Validation: ideally we would want to check the validity of the values passed at the CLI and fall back on our defaults (when present) if a value isn't valid...
• Instructions: I could add a "Coming soon!" subsection in the README to showcase that extra resources can be configured (even if the reveal template isn't published yet).

[SylvainCorlay]

This is looking good to me. I left an inline comment about the sort of recursive dict.extend that you are doing.

Small note: I rebased the PR on master and add a commit removing the prepare reveal-specific stage. Also, extra_resources is directly called resources.

[SylvainCorlay]

This looks good to me.

@maartenbreddels let me know if you have any comment on this before I merge. I would like to have a release including this so that @mkcor can iterate on the reveal template work.

[maartenbreddels]

LGTM, the failure was a timeout in nbconvert (we might want to increase that limit), restarted the travis job.

[SylvainCorlay]

OK, I pushed a commit removing the resources_reveal dictionary, which is specific to the reveal template.

(Since we can't elect any template-specific things to be part of the main voila package).

[maartenbreddels]

Good point. I think if templates want to have specific resources, we could add that to conf.json, at least for defaults.

[SylvainCorlay]

So the idea is that the reveal template should work without the resource dictionary providing theme, transition and scroll. In the voila template file, using reveal.get('theme', 'simple') instead of reveal.theme.

[mkcor]

Example use (update):

voila reveal.ipynb --template=reveal --VoilaConfiguration.resources="{'reveal': {'transition': 'fade'}}"