gistfile1.py

import os, sys

for subdir, dirs, files in os.walk(sys.argv[1]):
    print subdir
    for file in files:
        print file
        f = open(subdir+'/'+file, 'r')
        lines = f.readlines()
        f.close()
        f = open(subdir+'/'+file, 'w')
        for line in lines:
            newline = line.replace(sys.argv[2], sys.argv[3])
            f.write(newline)
        f.close()