finds matrix B from cumulative sum of given matrix A(mxn)

sum_matrix_auxillary.py

a = [[1,2,3,4],[3,4,5,6],[234,456,555,6789]]
b = [[0 for _ in range(len(a[0]))] for _ in range(len(a)) ]

def build_b(pos):
  i, j = pos
  if i ==0 and j == 0:
    b[i][j] = a[0][0]
    return b[i][j]
  elif b[i][j] !=0:
    return b[i][j]
  elif i == 0 and j < len(a[0]):
    b[i][j] = build_b((i,j-1)) + a[i][j]
  elif j == 0 and i < len(a):
    b[i][j] = build_b((i-1,len(a[0])-1)) + a[i][j]
  else:
    b[i][j] = build_b((i,j-1))+a[i][j]
    print(b[i][j], a[i][j])
  return b[i][j]
# for i in range(len(a)):
#   for j in range(len(a[0])):
#     b[i][j] = build_b((i,j))
build_b((2,3))
    

print(b)