python读取文件 求更简单、健壮代码

读取文件，假设只有 r w 两种方式，求优化一下代码！

import os

def rwfile(path, tag):
if os.path.exists(path):
if tag == "r" or tag == "w":
if tag == "r":
with open(path, tag) as f:
return f.readlines()

else:
with open(path, tag) as f:
return f.write("success")
else:
print "please enter tag value(\"r\" ? \"w\") "
else:
print "file path \"%s\" not exits" % path

if __name__ == '__main__':
print rwfile("../Ptext/test.txt", "r")