Python遍历某个文件夹下的所有文件夹，每个文件夹只保留最新7个文件

import os 
root = r"D:\_back" 
for dirpath, dirnames, filenames in os.walk(root): 
    for dirname in dirnames:
        _dir = os.path.join(dirpath,dirname)
        lists = os.listdir(_dir)
        lists = lists[::-1]
        save = 7
        if (len(lists) > save) :
            for i in range(save - 1, len(lists)-1):
                f = os.path.join(dirpath,dirname,lists[i])
                os.remove(f)
                print(f)