input: 
  def max_num(list):
      """Find and return the maximum number in a given list of integers.
      >>> max_num([9, 2, 5, 6, 4, 1])
      9
      >>> max_num([-4, -2, -5, -6, -4, -1])
      -1
      >>> max_num([0, 0, 0, 0, 0, 0])
      0
      """
      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