pstr[2:5]是截取内容，从索引2到索引4，不是5比如pstr="abcdefghijklmn"则pstr[2:5]就是"cde"不只是用于string，也可以用于list比如plist=["a","b","c","d","e","f","g"]则plist[2:5]就是["c","d","e"]
