How to relay the sum of each nested list to the next one?

My input is this list :

my_list = [[3, 4, -1], [0, 1], [-2], [7, 5, 8]]

I need to sum the nested list i and pad it to the right of the list i+1, it’s like a relay course.

I have to mention that the original list should be untouched and I’m not allowed to copy it.

wanted = [[3, 4, -1], [0, 1, 6], [-2, 7], [7, 5, 8, 5]]

I tried the code below but I got a list with more elements than my original one :

from itertools import pairwise

wanted = []

for left, right in pairwise(my_list):
    wanted.extend([left, right + [sum(left)]])
    
print(wanted)
[[3, 4, -1], [0, 1, 6], [0, 1], [-2, 1], [-2], [7, 5, 8, -2]]

Can you guys explain what’s happening please or suggest another solution ?

>Solution :

You can try itertools.accumulate:

from itertools import accumulate

my_list = [[3, 4, -1], [0, 1], [-2], [7, 5, 8]]

print(list(accumulate(my_list, lambda l1, l2: [*l2, sum(l1)])))

Prints:

[[3, 4, -1], [0, 1, 6], [-2, 7], [7, 5, 8, 5]]