Mergesort implementation in Python

mergesort.py

def merge(ls, left, mid, right):
  it1 = 0
  it2 = 0
  result = [0] * (right - left)
  while left + it1 < mid and mid + it2 < right:
    if ls[left + it1] < ls[mid + it2]:
      result[it1 + it2] = ls[left + it1]
      it1 += 1
    else:
      result[it1 + it2] = ls[mid + it2]
      it2 += 1
  while left + it1 < mid:
    result[it1 + it2] = ls[left + it1]
    it1 += 1
  while mid + it2 < right:
    result[it1 + it2] = ls[mid + it2]
    it2 += 1
  for i in range(it1 + it2):
    ls[left + i] = result[i]

def mergesort(ls, left, right):
  if left + 1 >= right:
    return
  mid = (left + right) / 2
  mergesort(ls, left, mid)
  mergesort(ls, mid, right)
  merge(ls, left, mid, right)

ls = [1, 0, 4, 2, 7, 8, 3, 5]
mergesort(ls, 0, len(ls))
print ls