OK, I've been running with the new code most the day, and it seems functional in my testing.

I only "sort of" follow your discussion about the "custom action class" caveat, probably because I haven't used "custom action classes"... I tried once, but failed to achieve my goal, as it was more ambitious than they presently support. If the [] value is significantly problematical in some manner, could positional nargs be set to a sentinal value that would avoid the assignment of the [] value? I realize that would require code changes in some other function or functions, in addition to the added new functions, so that would make the patch a bit more intrusive.

If _fallback helps some folks with understanding errors clearly, I won't object to it. I guess it would only be programmers that would be confused, because they would be the ones interpreting the errors... and with adequate testing, should fix the programming errors before the users get a chance to be confused.

So maybe your next .patch will be ready to ship!