len is a build-in function that returns the numbers of characters in a string:

Since we started counting at zero, the last index should be 5.

Alternatively, you can use negative indices, which count backward from the end of the string. The expression fruit[-1] yields the last letter, fruit[-2] yields the second to last, and so on.

The reason for index error is that no letter in ‘banana’ with the index -10. The first letter’s index should be fruit[-6].

from Thinking in Python