input: 
  def get_max_value(numbers):
      """
      Returns the largest value from a given list of numbers.
  
      >>> get_max_value([2, 4, 1, 6])
      6
      >>> get_max_value([10, 20, 15, 25])
      25
      """
      return max(numbers)
  
output:     if not numbers:
          return None