Бесконечный индекс массива

input = [5, 2, 13, 6, 9]
res_list = []
len_mass = len(input )
 
for i in range(len_mass):
    res_list += [input [i-1] + input [i+1]]

elements = [5, 2, 13, 6, 9]
length = len(elements)
 
for index in range(length):
   elements[index] = elements[index - 1] + elements[(index + 1) % length]
 
print(elements)

elements = [5, 2, 13, 6, 9]
elements_result = []
length = len(elements)
 
for index in range(length):
    elements_result.append(elements[index - 1] + elements[(index + 1) % length])
 
print(elements_result)

>>> initial = [5, 2, 13, 6, 9]
>>> result=[]
>>> for i,e in enumerate(initial):
...     prev=i-1 if i>1 else -1
...     next=i+1 if i<len(initial)-1 else 0
...     result.append(initial[prev]+initial[next])
...
>>> result
[11, 22, 8, 22, 11]

In [1]: a = [5, 2, 13, 6, 9]
 
In [2]: n = len(a)
 
In [3]: a[:] = [a[i - 1] + a[(i + 1) % n] for i in range(n)]
 
In [4]: print(*a)
11 18 8 22 11

lst = [5, 2, 13, 6, 9]
lst2=lst.copy()
lst.append(lst.pop(0))
lst2.insert(0,lst2.pop())
print([sum(i) for i in zip(lst, lst2)])

import numpy as np
lst =np.array( [5, 2, 13, 6, 9])
print(np.roll(lst,1)+np.roll(lst,-1))