Thank you Christopher for the heads up. Using paths as an example were really poor, they distract readers from the actual problem that is assembling a human readable string.
Maybe pointless, but on the basis of 30 seconds to run my test suite, see my failure, correct it, and run it again to be at the same point as if I didn't mistake, 300 workdays a year, spans 25 hours over 10 years, and I have already strategized my R&D to capitalize on python for another 10 years. So spending less than 25 hours on this would seem profitable, despite how pointless it is to actual programmers.
Anyway, at this point the proposal could also look like str.joinmap(*args, key=str). But I don't know, I can iterate on mapjoin for a while and open a new topic when I stabilize it. Meanwhile, I'm always happy to read y'all so feel free to keep posting :P
Have a great day