Switch templates via query string parameters

[jtpio]

A proof of concept for switching templates on the fly via the notebook metadata and query string parameters.

Related to #188 and #105.

• Rename the cli parameter template to templates to pass a list of available templates (instead of a single one)
• Fetch template from the notebook metadata
• Fetch template from the query string parameters
• (tests are not fixed yet)

One thing I'm not super happy with is the need to sort the nbconvert_template_paths list before passing it to the nbconvert exporter.

Also this approach might be completely invalidated if / when the template mechanism is changed for the next release.

[SylvainCorlay]

Thanks @jtpio!

I wonder if we could drop the list of allowed templates and just let users select any installed template from the url params.

The --template=foobar command-line argument would just select the default template that is used when none is specified with the url parameter.

[martinRenou]

I wonder if we could drop the list of allowed templates and just let users select any installed template from the url params.

👍

[jtpio]

Yes absolutely.

[jtpio]

Pushed new changes to:

• Drop the --templates=["gridstack", "default"] option and instead use all available templates
• Keep --template option to default to a specific template
• Assume there is always the default template installed

[martinRenou]

This sorted call feels bad to me... I suppose we should collect the right template paths for each request. See #258

[martinRenou]

I suppose something cleaner and safer would be templates = collect_template_paths(..., template_name=...)

[jtpio]

Agree it's not very elegant.
Would need to check how nbconvert handles templates that make use of inheritance.

[cailiang9]

It is cool but I think it is not a proper place to specify notebook specific template parameters.
Because the render link could be open to common users directly.
Allowing specify template from common users is unsafe.
A better place to specify notebook specific template is notebook metadata (edit -> edit metadata in jupyter).

[maartenbreddels]

Allowing specify template from common users is unsafe.

I think both options should be possible, but I agree that a user specifying a template should be possible to disable.

[maartenbreddels]

Closes #188

[jtpio]

Fixed by #637