Reversing letters in string without changing places of other characters :     def reversing(sentence):
    code = []
    for l1, l2 in zip(sentence, sentence[::-1]):
        if l1.isalpha() and l2.isalpha():
            l2 = l2.upper() if l1.isupper() else l2.lower()
        code.append(l2)
    return ''.join(code)

I need to reverse each letter in this sentence but every other character needs to stay in the same place as in the original sentence. This what I have so far but I'm stuck from here because I can't see what the mistake is. For example the sentence 'God bless our brave Confederates, Lord!' should be after this function 'Dro lseta red efnoc Evarbruossel, Bdog!' Thanks in advance.