What am I doing wrong?

def merge_sort(list, start = 0, end = None):
  """
  Sorts a list in ascending order
  Returns a new sorted list

  Divide: Find the midpoint for the list and divide into sublists
  Conquer: Recursively sort the sublists created in previous step
  Combine: Merge the sorted sublists created in previous step
  """
  a = []
  if end is None:
    end = len(list) - 1
  if start > end:
    return -1

  if start == end:
    a.append(list[start])
    return a

  left_start, left_end, right_start, right_end = split(list, start, end)
  left = merge_sort(list, left_start, left_end)
  right = merge_sort(list, right_start, right_end)

  return merge(left, right)

def split(list, start, end):
  if (end - start) == 1:
    return start, start, end, end
  left_start = start
  left_end = end // 2

  right_start = left_end + 1
  right_end = end
  #left = list[:mid]
  #right = list[mid:]

  return left_start, left_end, right_start, right_end

def merge(left, right):

  i = []
  r = 0
  l = 0

  while l < len(left) and r < len(right):
    if left[l] < right[r]:
      i.append(left[l])
      l += 1
    else :
      i.append(right[r])
      r += 1
  while l < len(left) :
    i.append(left[l])
    l +=1
  while r < len(right) :
    i.append(right[r])
    r +=1

  return i

def verify_sorted(list):
  n = len(list)
  if n == 0 or n == 1:
    return True
  return  list[0]<list[1] and verify_sorted(list[ 1:])


alist = [5, 3, 6, 9, 10, 15, 100, 50]
merge_sort(alist)
print(verify_sorted(alist))