Incorrect index returned from Python function

[dejohnny]

I have defined a procedure find_last, that takes as input two strings, a search string and a target string, that should return the last position in the search string where the target string is found. If the string is not found the procedure should return -1. Is there a way to fix this?

Python code:

def find_last(searched, target):
       first_pos = searched.find(target)
       size = len(searched)
       print size
       last_pos =1
       while last_pos <= size:
             #first_pos = searched.find(target)
              start_char = searched.find(target, first_pos)
              last_pos = searched.find(target, start_char + 1)
              last_pos += last_pos
              print last_pos
    
       return last_pos


print find_last('aaaa','a')