These sound like good refinements. You've been thinking.  By making the fallback happen externally, it simplifies the implementation of parse_intermixed_args, and forces the application to accept responsibility for calling it with a consistent set of arguments, or calling something else. I like that. I don't really see the fallback as a particularly useful feature, so pushing it outside the stdlib, yet still making it simple to implement for any that do find it to be useful, seems like a good tradeoff.