用Python替换文件中的字符串,递归地替换给定目录及其子目录中的字符串?

伪代码:

import os
import re
from os.path import walk
for root, dirs, files in os.walk("/home/noa/Desktop/codes"):
        for name in dirs:
                re.search("dbname=noa user=noa", "dbname=masi user=masi")
                   // I am trying to replace here a given match in a file