如何对依赖urllib2的模块进行单元测试？

urllib2的有一个调用的函数

build_opener()

install_opener()

urlopen()

import urllib2from StringIO import StringIOdef mock_response(req):    if req.get_full_url() == "http://example.com":        resp = urllib2.addinfourl(StringIO("mock file"), "mock message", req.get_full_url())        resp.pre = 200        resp.msg = "OK"        return respclass MyHTTPHandler(urllib2.HTTPHandler):    def http_open(self, req):        print "mock opener"        return mock_response(req)my_opener = urllib2.build_opener(MyHTTPHandler)urllib2.install_opener(my_opener)response=urllib2.urlopen("http://example.com")print response.read()print response.preprint response.msg