用python2.5 抓包时没有任何反馈，不清楚是什么原因？

Question

Python 2.5 (r25:51908, Sep 19 2006, 09:52:17) [MSC v.1310 32 bit (Intel)] on win32
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***************************************************************
Personal firewall software may warn about the connection IDLE
makes to its subprocess using this computer's internal loopback
interface. This connection is not visible on any external
interface and no data is sent to or received from the Internet.
****************************************************************

IDLE 1.2
下面用的是一个网上博客上经常出现的例子，但是没有任何反馈，停滞在循环中。系统是windows 8，wireshark中同样的过滤器能显示很多结果。

>>> import pcap,dpkt
>>> a=pcap.pcap()
>>> a.setfilter('tcp')
>>> for i,j in a:
    tem=dpkt.ethernet.Ethernet(j)
    print('%s %x',i,tem)