Good point - I had not conisdered the "N times" instruction. That would be very hard to accomplish with a zero lenght string!

Thank you, this function really doesn't work in this case. However, there is the following statement
in the task: "reverse function should be executed only N times", so this case is impossibe, but thank you again, I changed my solution.

I see this failing for a zero length string so this not be one of the test cases. Fix it simply...

return str if len(str) <= 1 else str[-1]+reverse(str[:-1])