def get(self, uid=None):
"""
获取通知，传入 uid: str 来指定获取用于指定用户的通知，缺省 uid=None 获取所有通知
"""
notices = []
for r in self.collection.find() or []:
n = Notice().from_dict(r)
if n.is_expired():
self.drop_expired(n)
if uid == None or n.is_for_user(uid):
notices.append(n.dumps())
return notices
