python 查找替换

[root@localhost ~]# less txt

123 345 678

accdfesdfdsf

[root@localhost ~]# less tihuan.py

#!/usr/bin/env python

import re

wenjian = open('txt','r+')

a = wenjian.read()

wenjian.seek(0,0)

f = (r'123')

b =re.findall(f,a)

if b != []:

   wenjian.write(a.replace(f,'zzh'))

   wenjian.close()

[root@localhost ~]# ./tihuan.py

[root@localhost ~]# less txt

zzh 345 678

accdfesdfdsf