Python max sub-array problem

gistfile1.py

from operator import add, sub
import sys

def max_subarray(arr):
    ret, cur = arr[0], 0 if arr[0] < 0 else arr[0]
    
    for num in arr[1:]:
        ret = max(ret, cur + num)
        cur = max(0, cur + num)
    return ret
    
def max_submatrix(matrix):
    n = len(matrix)
    sum_acc = [[0 for i in range(n)]]
    ret = -128
    
    for row in matrix:
        sum_acc.append(map(add, sum_acc[-1], row))
    
    for i in range(1, n + 1):
        for j in range(i, n + 1):
            tmp = map(sub, sum_acc[j], sum_acc[i - 1])
            ret = max(ret, max_subarray(tmp))
    return ret
    
if __name__ == '__main__':
    n = sys.stdin.readline()
    n = int(n)
    matrix = []
    for i in range(n):
        nums = sys.stdin.readline()
        nums = map(lambda x: int(x), nums.split())
        matrix.append(nums)
    
    print '%d\n' % (max_submatrix(matrix))