Utility mechanism for plugins to render templates

Sometimes a plugin will need to render a template for some custom UI. We need a documented API for doing this, which ensures that everything will work correctly if you extend base.html etc.

See also #576. This could be a .render() method on the Datasette class, but that feels a bit weird - should that class also take responsibility for rendering?

[simonw]

For the moment I’m going to move it to async def render_template() on datasette but otherwise keep the implementation the same.

The new signature will be:

async def render_template(self, template, context=None, request=None, view_name=None):

template can be a list of strings or a single string. If a list of strings a template will be selected from them.

I’ll reconsider the large list of default context variables later on in a separate ticket.

[simonw]

The code in question currently lives in BaseView.render():

https://github.com/simonw/datasette/blob/d54318fc7f2565e6121920ce1ea9cb8b700e629a/datasette/views/base.py#L106-L163

Should datasette.render_template() do exactly this, or should it be slightly different?

Plugins need the option to not pass a request object - so maybe that parameter becomes optional.

Perhaps plugins should be able to render templates without other plugins getting to inject their own variables?

Does it always make sense to dump in all of those extra template context variables?