python - 关于urllib2的add_handler

请问怎么o2就成了None了，不应该跟o一样是urllib2.OpenerDirector instance吗？add_handler使用的例子不太好找，难道我理解的用法不对？

Python 2.7.3 |EPD_free 7.3-2 (32-bit)| (default, Apr 12 2012, 14:30:37) [MSC v.1500 32 bit (Intel)] on win32 
Type "credits", "demo" or "enthought" for more information.                                                  
>>> import urllib2                                                                                           
>>> o = urllib2.build_opener()                                                                               
>>> o                                                                                                        
                                                              
>>> p = {'http':'127.0.0.1:8080'}                                                                            
>>> ph = urllib2.ProxyHandler(p)                                                                             
>>> ph                                                                                                       
                                                                
>>> o2 = o.add_handler(ph)                                                                                   
>>> o2                                                                                                       
>>>