Replacing elements of one list with another list in Python

I have two lists J1,J2 containing lists. I want to replace and insert elements of J2 into J1. But I am getting an error. I present the expected output.

import numpy as np

J1=[[0, 0, 0, 0, 0, 9, 0]]

J2=[[1,4]]
for i in range(0,len(J2)):
    J=[J2[i][j] for j in J1]
    print(J)

The error is

in <listcomp>
    J=[J2[i][j] for j in J1]

TypeError: list indices must be integers or slices, not list

The expected output is

[[1, 4, 0, 0, 0, 9, 0]]

The error was caused by the fact that, in your code, j is an element of J1, more specifically, the first time, j = [0, 0, 0, 0, 0, 9, 0]. This is indeed a list, and not an integer or a slice, as requested by J2[i][j].

This should work, in your case where J2 is shorter than J1:

J1=[[0, 0, 0, 0, 0, 9, 0]]

J2=[[1,4]]
for i in range(0,len(J2[0])):
    J1[0][i] += J2[0][j]

print(J1)

For a more general approach:

J1 = [[1, 4]]
J2 = [[0, 0, 0, 9]]

index = 0
J = [[]]
if __name__ == "__main__":
    while index < len(J1[0]) and index < len(J2[0]):
        J[0].append(J1[0][index] + J2[0][index])
        index += 1
        if index == len(J1[0]):
            J[0].extend(J2[0][index:])
        elif index == len(J2[0]):
            J[0].extend(J1[0][index:])

    print(J)

If you want to unpack each element instead of calling the list element from the index, you can use zip_longest from itertools. The function zip_longest will zip to the longest list, and fill in null values for the shorter list once the last element has been unpacked. The zip function from python standard library stops at the shortest length list.

Also, we can implement recursion, so that it can handle any dimension of lists.

from itertools import zip_longest

J1=[[0, 0, 0, 0, 0, 9, 0]]
J2=[[1,4]]

def replace_list(J1,J2):
    j = []
    for j1,j2 in zip_longest(J1, J2, fillvalue=[]):
        if not isinstance(j1,int) and not isinstance(j2,int): # When J2 and J1 are a inner list
            j.append(replace_list(j1,j2))
        elif not j2: # When J2 is shorter than J1 
            j.append(j1)
        else:
            j.append(j2)
    return j

J = replace_list(J1,J2)
print(J) # [[1, 4, 0, 0, 0, 9, 0]]

# This can handle uneven lists in multiple dimensions as well.
J1=[
    [0, 0, 0, 0, 0, 9, 0],
    [1, 2, 3, 4, 0, 0, 0, 1]
    ]
J2=[
    [1,4],
    [9, 8, 7, 6],
    [1, 1, 1, 1, 1, 1, 1]
    ]
    
J = replace_list(J1,J2)
print(J) # [[1, 4, 0, 0, 0, 9, 0], [9, 8, 7, 6, 0, 0, 0, 1], [1, 1, 1, 1, 1, 1, 1]]