python的单例模式

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

class NBSite(object):
    def __init__(self):
        self.name = 'nb'
        self.namespace = 'nb'
        self._registry= {}
    def register(self,model,model_nb=None):
        if not model_nb:
            model_nb=ModelNB
        self._registry[model]=model_nb(model,self)
        print(self._registry)
    def login(self,request):
        return HttpResponse('登陆页面')
    def logout(self,request):
        return HttpResponse('注销页面')


    def get_urls(self):
        patterns=[]
        from django.conf.urls import url
        patterns +=[
            url(r'^login/',self.login),
            url(r'^logout/',self.logout),
        ]
        for model_class,model_nb_obj in self._registry.items():
            patterns +=[
                url(r'^%s/%s/' %(model_class._meta.app_label,model_class._meta.model_name,),
                    model_nb_obj.urls)]
        return patterns
    @property
    def urls(self):
        return self.get_urls(),self.name,self.namespace

site=NBSite()