I find the reason: the notepad++ line num(left side) shows 6, but open().readlines() return list ['aaa\n','bbb\n','ccc\n','\n','\n']
'\n' will add a new line num in editor,but not add a list num
So if the last line is like 'bbb',the script is correct if the last is like '\n' ,'\n',the line num must add 1