所谓单例，是指一个类的实例从始至终只能被创建一次。

方法1

使用类在初始化时候被实例

1 class NBSite(object): 2     def __init__(self): 3         self.name = 'nb' 4         self.namespace = 'nb' 5         self._registry= {} 6     def register(self,model,model_nb=None): 7         if not model_nb: 8             model_nb=ModelNB 9         self._registry[model]=model_nb(model,self)10         print(self._registry)11     def login(self,request):12         return HttpResponse('登陆页面')13     def logout(self,request):14         return HttpResponse('注销页面')15 16 17     def get_urls(self):18         patterns=[]19         from django.conf.urls import url20         patterns +=[21             url(r'^login/',self.login),22             url(r'^logout/',self.logout),23         ]24         for model_class,model_nb_obj in self._registry.items():25             patterns +=[26                 url(r'^%s/%s/' %(model_class._meta.app_label,model_class._meta.model_name,),27                     model_nb_obj.urls)]28         return patterns29     @property30     def urls(self):31         return self.get_urls(),self.name,self.namespace32 33 site=NBSite()