def change(string):
    for i in range(16):
        helper = string[i]
        string[i]=string[15-i]
        string[15-i]=helper
    return string
print (change("abcdefghijklmnop"))

string[i]=helper2[0]
TypeError: 'str' object does not support item assignment

def change(input_str):
  return input_str[-1] + input_str[1:-1] + input_str[0]

def change(string):
    string = [c for c in string]
    for i in range(16):
        helper = string[i]
        string[i]=string[15-i]
        string[15-i]=helper
    return string
print (change("abcdefghijklmnop"))

def change(string):
    return string[-1]+string[1:-1]+string[0]

my_list = [1,2,3,4,5,6,7,8,9]
a, *middle, b = my_list
my_new_list = [b, *middle, a]
my_list
[1, 2, 3, 4, 5, 6, 7, 8, 9]
my_new_list
[9, 2, 3, 4, 5, 6, 7, 8, 1]