我们在平常的工作中有时候需要对摸一个文件进行操作，比如在一个文件的每行前面添加##之类的，在shell中这个需求很简单，用sed单行就能搞定，下面我们来看看一个文件：

[root@host-192-168-209-128 py-sed]# cat a.txt
this is a text
this is use for python
this is also user for sed
this is a end test file
[root@host-192-168-209-128 py-sed]#
用sed的单行命令来搞定这个需求很简单，看下代码：
[root@host-192-168-209-128 py-sed]# sed 's/^/##/g' a.txt
##this is a text
##this is use for python
##this is also user for sed
##this is a end test file
[root@host-192-168-209-128 py-sed]#

看看，果然够强大的sed啊，下面我来给大家介绍介绍如何用python实现这个有时候经常需要的操作，直接上代码了：
[root@host-192-168-209-128 py-sed]# cat a.py
#!/usr/bin/env python

with open('a.txt') as f:
       con=f.readlines()
       for i in range(0,len(con)):
               print "###"+con[i].rstrip('\n')
代码实在很简单，看看效果如何吧：
[root@host-192-168-209-128 py-sed]# python a.py
###this is a text
###this is use for python
###this is also user for sed
###this is a end test file

呵呵，效果出来了吧，但是稍有缺陷，这个需要操作的对象文件我们是写死在代码里面的，如何把文件名作为参数传递给脚本呢，我们需要修改，以实现如下几个功能：
1. 需要把操作的文件作为参数传给脚本
2.需要对操作的对象进行判断，是否存在
3.如果脚本运行错误，需要有友好的提示效果
基于以上的需求，给出代码的最终版本,代码如下：

[root@host-192-168-209-128 py-sed]# cat tou.py
#!/usr/bin/env python
'''
edit by qhz
Email : [email protected]
This scrip to add "###" at every line for file

'''
def usage():
       print   '''
===============================================
This script to add "###" at every line for file
Use Example:
python script.py file
===============================================
'''
import sys
import os
if len(sys.argv) == 2:
        if os.path.isfile(sys.argv[1]):
                with open(sys.argv[1]) as f:
                       con=f.readlines()
                       for i in range(0,len(con)):
                              print '###'+con[i].strip('\n')
      else:
                print "==============================================="
                 print "Your input file name is not exit or not correct"
                 print "Please try again ,bye ..."
                 print "==============================================="
else:
          usage()
          exit()
[root@host-192-168-209-128 py-sed]#

下面来看看各种情况和效果：
[root@host-192-168-209-128 py-sed]# python tou.py

===============================================
This script to add "###" at every line for file
Use Example:
python script.py file
===============================================

[root@host-192-168-209-128 py-sed]# python tou.py a.tx
===============================================
Your input file name is not exit or not correct
Please try again ,bye ...
===============================================
[root@host-192-168-209-128 py-sed]# python tou.py a.txt
###this is a text
###this is use for python
###this is also user for sed
###this is a end test file
[root@host-192-168-209-128 py-sed]#


    好了，这次的python介绍就到这里，我将为大家陆续模拟一些sed的简单功能，希望大家能喜欢