input: Write a function to get the maximum and minimum values in a given list. If the list is empty, return (None, None).
  def get_max_min(lst):
      if not lst:
          return (None, None)
      min_value = lst[0]
      max_value = lst[0]
      
      for i in range(1, len(lst)):
          if lst[i] > max_value:
              max_value = lst[i]
          if lst[i] < min_value:
              min_value = lst[i]
      
      return (min_value, max_value)
  
  assert get_max_min([3, 5, 7, 1, 9]) == (1, 9)
  assert get_max_min([10, 2, 8, 4, 6]) == (2, 10)
  assert get_max_min([-1, -5, -3, -2, -4]) == (-5, -1)
  assert get_max_min([]) == (None, None)
output: def get_max_min(lst):
      if not lst:
          return (None, None)
      min_value = lst[0]
      max_value = lst[0]
      
      for i in range(1, len(lst)):
          if lst[i] > max_value:
              max_value = lst[i]
          if lst[i] < min_value:
              min_value = lst[i]
      
      return (min_value, max_value)