python练习程序（读写文件）

import os

file=open("123.txt","r")
for line in file.readlines():
    print line,
file.close()

file=open("123.txt","w")
file.write("sdfsdfsdf")
file.close()

file=open("123.txt","a")
file.write("erertyerty")
file.close()

file=open("123.txt","r")
for line in file.readlines():
    print line,
file.close()