[Python] Forming a frame of zeros around a matrix in python

I am trying to pad a matrix with zeros, but am not really sure how to do it. Basically I need to surround a matrix with an n amount of zeros. The input matrix is huge (it represents an image)

Example:

Input:
1 2 3 4
5 6 7 8
4 3 2 1

n = 2

Output:
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 1 2 3 4 0 0
0 0 5 6 7 8 0 0
0 0 4 3 2 1 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0


The problem is that I get "k" is not accessed and "l" is not accessed.

Code:

import numpy as np

n = 2

matrix = [[1, 2, 3, 4],
[5, 6, 7, 8],
[4, 3, 2, 1]]
modified_matrix = np.zeros(shape=((len(matrix) + n), (len(matrix[0]) + n)), dtype=int)

k = n
l = n
modified_matrix = [[l] for l in range(len(matrix[k])] for k in range(len(matrix))]

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