enable extensions to override default templates

[MJochim]

This is more or less a follow-up to #1643 and #1658, both of which change how extensions’ templates are handled.

I would like to be able to override Fava’s default templates (e.g. _journal_table.html) in my extension. I currently achieve this as outlined in the next paragraph, making use of the before_request hook I have proposed in PR #1685. My proposal in this issue is to do that globally for all extensions, so that an extension author can simply drop a file templates/_journal_table.html in their extension without resorting to a hook.

PR #1643 used the create_app function to replace fava_app.jinja_loader with a ChoiceLoader that included both Fava’s default loader as well as an extension loader, but only in the extension report endpoint. My private plugin, on the other hand, uses the before_request hook to replace fava_app.jinja_loader with a similar type of ChoiceLoader, but for all endpoints. In #1643, Fava’s default loader took precedence over the extension loader, but in my private plugin, the extension loader takes precedence. The code of my private plugin currently looks like this:

def before_request(self):
    if not current_app.jinja_loader:
        raise ValueError("Expected Flask app to have jinja_loader.")
    ext_loader = jinja2.FileSystemLoader(self.extension_dir / "templates")
    loader = jinja2.ChoiceLoader([ext_loader, current_app.jinja_loader])
    current_app.jinja_loader = loader

It would be possible to do something like the following, e.g. in applicaton.py/_perform_global_filters():

if not current_app.jinja_loader:
    raise ValueError("Expected Flask app to have jinja_loader.")

all_loaders = [
    jinja2.FileSystemLoader(x.extension_dir / "templates")
    for x in ledger.extensions._exts # is there a non-private API to get the content of _exts?
]

all_loaders.append(current_app.jinja_loader)

current_app.jinja_loader = jinja2.ChoiceLoader(all_loaders)

This would not interfere with the changes in #1658. It would be a little strange in the extension report endpoint, though: That endpoint prepends the respective extension’s FileSystemLoader again, making it appear twice in the list; but usefully also making sure that it takes precedence over the others.

If you like this proposal, I would prepare a pull request.

[yagebu]

Mhh, this would be too intrusive for my taste, as it basically the "allow extension to override any function" equivalent in HTML templates. I do not want to maintain compatibility for these templates and in particular not in this direction. So an extension such functionality could/would break very easily and I do not want to provide an easy way to do this.