Iterating through directories with Python

I need to iterate through the subdirectories of a given directory and search for files. If I get a file I have to open it and change the content and replace it with my own lines.

I tried this:

import os

rootdir="C:/Users/sid/Desktop/test"

for subdir, dirs, files in os.walk(rootdir):
    for file in files:
        f=open(file,'r')
        lines=f.readlines()
        f.close()
        f=open(file,'w')
        for line in lines:
            newline = "No you are not"
            f.write(newline)
        f.close()

but I am getting an error. What am I doing wrong?