>>> f = open(r'E:\python\somefile.txt','w')    打开文件，写模式
>>> f.write('this\nis no \nhailu')       写入三行话
17
>>> f.close()
>>> f = open(r'E:\python\somefile.txt','r')
>>> f.read()
'this\nis no \nhailu'          查看一下
>>> f = open(r'E:\python\somefile.txt')
>>> lines = f.readlines()         把每一行的内容变为集合lines 的一个元素

>>> f.close()
>>> lines[1] = "isn't a\n"         给lines的第二个元素 重新赋值（改写了）
>>> f = open(r'E:\python\somefile.txt','w')
>>> f.writelines(lines)
>>> f.close()             
>>  

<pre name="code" class="python">this
isn't a
hailu