input: 
  def max_num(list):
      """
      Find and return the maximum number from a given list.
      >>> max_num([9, 2, 5, 6, 4, 1])
      9
      >>> max_num([-1, -5, -3, -2])
      -1
      """
      max_num = list[0]
      for num in list:
          if num > max_num:
              max_num = num
      return max_num
  
output:     max_num = list[0]
      for num in list:
          if num > max_num:
              max_num = num
      return max_num