'M*N matrix'
[[0 1 0]
 [2 0 1]
 [0 4 0]
 [0 0 0]]

def sum_neighbors(A, i, j):
    rows, columns = A.shape
    r0, r1 = max(0, i-1), min(rows-1, i+1)
    c0, c1 = max(0, j-1), min(columns-1, j+1)
    rs = list({r0, i, r1})
    cs = [[c] for c in list({c0, j, c1})]

    return A[rs, cs].sum() - A[i, j]

import numpy as np

mxn = np.array([[0, 1, 0],
                [2, 0, 1],
                [0, 4, 0],
                [0, 0, 0]])

for i, j in [(0, 0), (1, 0), (1, 1)]:
    s = "sum of neigbors for i={} and j={} is {}"
    print s.format(i, j, sum_neighbors(mxn, i, j))

sum of neigbors for i=0 and j=0 is 3
sum of neigbors for i=1 and j=0 is 5
sum of neigbors for i=1 and j=1 is 8