input: 
  def max_num(list):
      """Optimize the following Python program to find the maximum number in a list.
      >>> max_num([9, 2, 5, 6, 4, 1])
      9
      >>> max_num([-1, -3, -5, -2, -4])
      -1
      >>> max_num([10])
      10
      """
  
output:     if not list:
          return None
      max_value = list[0]
      for num in list:
          if num > max_value:
              max_value = num
      return max_value