One problem here is that when you're doing s.lower() you're not assigning that to anything. Doing s.lower() does not change s; it returns a new string with all letters lowercase. To save that new string you should assign it. If you print your list_s you'll see that the first 'T' remains uppercase. You're getting away in the test case because there's another copy of 't', but that won't happen for other test cases.