def DiffValueLists(new_list, old_list):
  """Give an old list and a new list, return the added and removed items."""
  if not old_list:
    return new_list, []
  if not new_list:
    return [], old_list

  added = []
  removed = old_list[:]  # Assume everything was removed, then narrow that down
  for val in new_list:
    if val in removed:
      removed.remove(val)
    else:
      added.append(val)

  return added, removed