Maximum Independent weight and traceback set in Graph

The following code shall find the maximum weight independent set and weight in a given graph:

def wis(arr):
 if(len(arr) == 0):
  return 0
 A = []
 B = {}
 for i in xrange(len(arr)+1):
  A.insert(i, 0)
 A[0] = 0
 A[1] = arr[0]
 B[arr[0]] = [arr[0]]
 for i in xrange(2, len(arr)+1): 
  A[i] = max(A[i-1], A[i-2]+arr[i-1])
  try:
   if(A[i-2]+arr[i-1] > A[i-1]):
    B[A[i]] = [B[A[i-2]], arr[i-1]]
  except KeyError:
   B[A[i-2]] = 0
   B[A[i]] = [B[A[i-2]], arr[i-1]]
 return A,B

res, trace = wis([5, 7, 100, -15, 3, 2])
print res[len(res)-1]
print trace[res[len(res)-1]]