One thing I've done in this implementation is to build up a string to pass to str.format(), instead of using the original string. This new string uses positional parameters instead of named parameters.

I had originally proposed to add a string.interpolate() to do the heavy lifting here, which would have meant I could use the original string (as seen in the source code), and not build up a new string and pass it to str.format(). I still might do that, but for now, the approach using str.format() is good enough.