#!/usr/bin/env python3

# turn a string into lower case;
# tests immutability of strings

import string

s0="Hello World"
print(s0)
# t=""  # this doesn't work, because strings are immutable
t=[]
for c in s0:
    if c.isalpha() and (c in string.ascii_uppercase):
        n=string.ascii_uppercase.index(c)
        # s[n] = string.ascii_lowercase[n] # NO: str immutable
        t.append(string.ascii_lowercase[n])
    else:  
        t.append(c)

s1="".join(t)  # list-to-string
print(s1)