"Increment" a string of uppercase characters
A slight simplification of the if else structure is possible:

def increment_str(s):
    lpart = s.rstrip('Z')
    num_replacements = len(s) - len(lpart)
    new_s = lpart[:-1] + increment_char(lpart[-1]) if lpart else 'A'
    new_s += 'A' * num_replacements
    return new_s