Possible solution using list comprehension:

alpha = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
''.join(alpha[i % 26] for i in range(18,18+13))

Out[104]: 'STUVWXYZABCDE'

Or per the original post:

a = [1, 2, 3, 4, 5, 6]
[a[i % len(a)] for i in range(4, 4 + 5)]

Out[106]: [5, 6, 1, 2, 3]

Thanks to Andy Richter for the expanded versions that led to this idea.