input: Write a function to calculate the maximum, minimum, and mean number of elements in a given string. If the input string is empty, return (None, None, None).
  
  def get_stats_for_string(string):
      if not string:
          return (None, None, None)
      num_elements = len(string)
      max_element = max(string)
      min_element = min(string)
      mean_element = sum(ord(char) for char in string) / num_elements
      return (max_element, min_element, mean_element)
  
  assert get_stats_for_string("") == (None, None, None)
  assert get_stats_for_string("a") == ('a', 'a', 97.0)
  assert get_stats_for_string("abc") == ('c', 'a', 98.0)
  assert get_stats_for_string("hello") == ('o', 'e', 105.2)
  assert get_stats_for_string("world") == ('w', 'd', 108.4)
output: def get_stats_for_string(string):
      if not string:
          return (None, None, None)
      num_elements = len(string)
      max_element = max(string)
      min_element = min(string)
      mean_element = sum(ord(char) for char in string) / num_elements
      return (max_element, min_element, mean_element)