I am at the "write a function that reverses a string without creating a new string" question and this is the answer on the site:

  def reverse(str):
    left_ptr = 0
    right_ptr = len(str) - 1
    middle = len(str) / 2
    while left_ptr <= middle:
        # swap
        temp = str[left_ptr]
        str[left_ptr] = str[right_ptr]
        str[right_ptr]= temp

wouldn't this create an infinite loop? You're never incrementing / decrementing left_ptr or right_ptr.