Are you certain that Transmission doesn't support commas in labels? Also I would update the docstring.

Are you certain that Transmission doesn't support commas in labels?

Yes

I'm good with this, the only thing I'm dubious about is the string formatted comma delimited multi-label configuration. I think we should just require multiple labels to be in a list.
i.e. Instead of this:

      labels: "Source: xyz, Series: {{series_name}}, Quality: {{quality}}"

require this:

      labels:
        - "Source: xyz"
        - "Series: {{series_name}}"
        - "Quality: {{quality}}"

So, is concatenating labels from config and entry ok, since it's goes against how that usually works?

So, is concatenating labels from config and entry ok, since it's goes against how that usually works?

Yeah, I guess that's good. Seems like it's a bit more flexible than just overriding config labels with entry labels.

Are certain fields always available in templates?
Can "quality" not be there?

The documentation says that it will produce an empty string, but I noticed that invalid fields cause an exception.
And in code, it actually just captures the exception and logs the error.

It should probably be done here the same.
And should the label just be ignored or left unrendered?

The documentation says that it will produce an empty string, but I noticed that invalid fields cause an exception.
And in code, it actually just captures the exception and logs the error.

Yeah, that documentation is probably oversimplified/innacurate depending on where templates are being used. I'd say we should catch errors and log a warning then ignore that label for ones that the rendering failed here.

Looks good. Thanks for this!