input: def get_max_value(numbers: list) -> int:
      """Return the largest value from a given list of numbers.
      >>> get_max_value([2, 4, 1, 6])
      6
      >>> get_max_value([7, 3, 9, 5])
      9
      """
output:     if not numbers:
          return None
      max_value = numbers[0]
      for num in numbers:
          if num > max_value:
              max_value = num
      return max_value