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.

what mess formats!