Why does Python return negative list indexes?

Question

If I have this list with 10 elements:

>>> l = [1,2,3,4,5,6,7,8,9,0]

Why will l[10] return an IndexError, but l[-1] returns 0?

Answer 1

It's because l[-1] is equal to l[len(l)-1], similarly l[-2] is equal to l[len(l)-2]

>>> lis=[1,2,3,4,5]
>>> lis[-1],lis[-2],lis[-3]
(5, 4, 3)
>>> lis[len(lis)-1],lis[len(lis)-2],lis[len(lis)-3]
(5, 4, 3)