1. 数字类型：int、complex、float

    布尔类型：True、False
    字符串类型：格式化、切片

    1. show you the code

      1. while True:
      2.  s = input('Please input a str within length of 20: ')
      3.  if len(s) > 20:
      4.      print('The str you have inputted exceed the length limit!', end='')
      5.  else:
      6.      print("Length: %d, reversed: %s" % (len(s), s[::-1]))

    2. '3'+'4'的结果是'34'