If you want to include testing for default arguments, or optional arguments, that's fine; but in that case it might be a good idea to mention this in the instructions...

I can't change main idea. Only translate. Probably this test show your understanding default arguments.

Not exactly... when python creates a list (or string), it automatically saves its length. Finding the length is a single operation, not a loop through its elements. So no need to save it, and it's always O(1) complexity, no matter the length itself.

read more comments here:
https://stackoverflow.com/questions/699177/python-do-python-lists-keep-a-count-for-len-or-does-it-count-for-each-call

hitting a recursion limit is a problem of your solution