Finding missing items in an int list

Here is a problem I am trying to solve: trying to find missing photographs from a sequence of filenames. The problem boils down to: given an unsorted list of integers, return a sorted list of missing numbers. Below is my code.

What I am looking for are:

• Are there more efficient algorithms?
• Is there any performance problem if the list is large (tens of thousands)

• Corner cases which does not work?

  def find_missing_items(int_list):
      '''
      Finds missing integer within an unsorted list and return a list of 
      missing items
  
      >>> find_missing_items([1, 2, 5, 6, 7, 10])
      [3, 4, 8, 9]
  
      >>> find_missing_items([3, 1, 2])
      []
      '''
  
      # Put the list in a set, find smallest and largest items
      original_set  = set(int_list)
      smallest_item = min(original_set)
      largest_item  = max(original_set)
  
      # Create a super set of all items from smallest to largest
      full_set = set(xrange(smallest_item, largest_item + 1))
  
      # Missing items are the ones that are in the full_set, but not in
      # the original_set
      return sorted(list(full_set - original_set))
  
  if __name__ == '__main__':
      import doctest
      doctest.testmod()