How to make string indices circular when string index out of range

Suppose I have this string:

myString = 'abcdefghijklmnopqrstuvwxyz'

If I ask for myString[26], I will get an error saying string index out of range. Only myString[0] through myString[25] are valid.

My desired result is to ask for myString[26] and retu 'a', because I have overreached the string by 1, and come back around front at 'a' in a circular motion. How can I achieve this?