So, if the 1st arg is optional, how does one call this function to pass the 2nd and 3rd args?
To me, only the 3rd arg is truly optional. This seems to be confirmed by the function signature definition on the link provided in the question:
range(start, stop[, step])
I can see how Python allows passing just one arg and infer that refers to stop.
Nevertheless, the 1st arg (start) seems "semi-optional" at best.