python subprocess 模块子进程输出问题：报错

这段代码是想在获取子进程里面的实时的标准输出结果，而不是等子进程结束以后再输出最后的结果，但是每次执行输出的结果都不一样，比较费解，求大神指点？感谢！运行环境ubuntu 12.04.3 64bit

#!/usr/bin/python
import subprocess
def run_cmd_redis(cmd):
    p = subprocess.Popen(cmd, shell=True, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
    res = p.returncode
    while res is None:
            line = p.stdout.readline()
            res = p.poll()
            print p.returncode
            print line


run_cmd_redis('ifconfig')

root@demo1 :~/# ./test.py 
None
eth0      Link encap:Ethernet  HWaddr 00:0c:29:28:b5:d3  


None
          inet addr:10.1.35.226  Bcast:10.1.35.255  Mask:255.255.255.0


None
          inet6 addr: fe80::20c:29ff:fe28:b5d3/64 Scope:Link


None
          UP BROADCAST MULTICAST  MTU:1500  Metric:1


None
          RX packets:92 errors:0 dropped:10 overruns:0 frame:0


None
          TX packets:6 errors:0 dropped:0 overruns:0 carrier:0


None
          collisions:0 txqueuelen:1000 


None
          RX bytes:6274 (6.2 KB)  TX bytes:468 (468.0 B)


0




root@demo1 :~/# ./test.py 
None
eth0      Link encap:Ethernet  HWaddr 00:0c:29:28:b5:d3  


0
          inet addr:10.1.35.226  Bcast:10.1.35.255  Mask:255.255.255.0


root@demo1 :~/# ./test.py 
None
eth0      Link encap:Ethernet  HWaddr 00:0c:29:28:b5:d3  


0
          inet addr:10.1.35.226  Bcast:10.1.35.255  Mask:255.255.255.0


root@demo1 :~/# ./test.py 
None
eth0      Link encap:Ethernet  HWaddr 00:0c:29:28:b5:d3  


None
          inet addr:10.1.35.226  Bcast:10.1.35.255  Mask:255.255.255.0


None
          inet6 addr: fe80::20c:29ff:fe28:b5d3/64 Scope:Link


None
          UP BROADCAST MULTICAST  MTU:1500  Metric:1


None
          RX packets:92 errors:0 dropped:10 overruns:0 frame:0


None
          TX packets:6 errors:0 dropped:0 overruns:0 carrier:0


None
          collisions:0 txqueuelen:1000 


None
          RX bytes:6274 (6.2 KB)  TX bytes:468 (468.0 B)


None




None
eth1      Link encap:Ethernet  HWaddr 00:0c:29:28:b5:dd  


None
          inet addr:10.1.9.226  Bcast:10.1.9.255  Mask:255.255.255.0


None
          inet6 addr: fe80::20c:29ff:fe28:b5dd/64 Scope:Link


None
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1


None
          RX packets:27569982 errors:0 dropped:1038 overruns:0 frame:0


None
          TX packets:27813981 errors:0 dropped:0 overruns:0 carrier:0


None
          collisions:0 txqueuelen:1000 


None
          RX bytes:2791401044 (2.7 GB)  TX bytes:5211953339 (5.2 GB)


None




None
lo        Link encap:Local Loopback  


None
          inet addr:127.0.0.1  Mask:255.0.0.0


None
          inet6 addr: ::1/128 Scope:Host


None
          UP LOOPBACK RUNNING  MTU:65536  Metric:1


None
          RX packets:17800715 errors:0 dropped:0 overruns:0 frame:0


None
          TX packets:17800715 errors:0 dropped:0 overruns:0 carrier:0


None
          collisions:0 txqueuelen:0 


None
          RX bytes:1538890210 (1.5 GB)  TX bytes:1538890210 (1.5 GB)


None




None


None


None


None


None


None


None


None


None


None


None


None


None


None


None


None


None


None


None


None


None


None


None


None


None


None


None


None


None


None


None


0