python - 如何读取通过cookielib.MozillaCookieJar()保存的cookies?

def save_cookie():
      cookie = cookielib.MozillaCookieJar('cookie.txt')
      opener = urllib2.build_opener(HTTPCookieProcessor(cookie))
      req = urllib2.Request(url=targeturl,headers=targetheaders)
      opener.open(req)
      cookie.save(ignore_discard=True,ignore_expires=True)

我想通过cookielib.MozillaCookieJar('cookie.txt').load()来读取所保存的cookies，
但发现返回值是None!