从python读取指令

Question

我有此代码：

import os
def inplace_change(filename, old_string, new_string):
    # Safely read the input filename using 'with'
    with open(filename, 'r') as f:
        s = f.read()
        if old_string not in s:
            print('"{old_string}" not found in {filename}.'.format(**locals()))
            return
        else:
        # Safely write the changed content, if found in the file
            with open(filename, 'w') as f:
                s = s.replace(old_string, new_string)
                f.write(s)

path = raw_input("Enter the file's full path: ")  
old = raw_input("String to change: ")
new = raw_input("change to: ")

print "********************************"
print "**** WORKING... PLEASE WAIT ****"
print "********************************\n"
for file in os.listdir(path):
    filename = os.path.join(path,file)
    inplace_change(filename, old, new)

os.system("pause")

如您所见，代码将文件中的子字符串替换为另一个子字符串。我希望我的代码按照文本文件中的指示更改文本，例如“指令文件”要更改的内容。 文本文件将是：

"old_string" "new_string"
"old_string" "new_string"
"old_string" "new_ string"

结果将是目录中的所有文件将所有old_string更改为new_string

我该怎么办？