Use the %
operator to produce a modulus:
notes[note % len(notes)]
Demo:
>>> notes = ["a", "a#", "b", "c", "c#", "d", "e", "f", "f#", "g", "g#"]
>>> note = 21
>>> notes[note % len(notes)]
'g#'
or in a loop:
>>> for note in range(22):
... print notes[note % len(notes)],
...
a a# b c c# d e f f# g g# a a# b c c# d e f f# g g#