python - NoneType 没有属性 Append

Question

我是 Python 新手。我不明白为什么变量在我的代码中的某个点是 None:

class UsersInRoom(webapp.RequestHandler):
    def get(self):
        room_id = self.request.get("room_id")
        username = self.request.get("username")
        UserInRoom_entities = UserInRoom.gql("WHERE room = :1", room_id).get()
        if UserInRoom_entities:
            for user_in_room in UserInRoom_entities:
                if user_in_room.username == username:
                    user_in_room.put() # last_poll auto updates to now whenenever user_in_room is saved
        else:
            user_in_room = UserInRoom()
            user_in_room.username = username
            user_in_room.put()
            // error here, on line 160
            UserInRoom_entities = []
            UserInRoom_entities.append(user_in_room)

        # name is `user_at_room` intead of `user_in_room` to avoid confusion    
        usernames = [user_at_room.username for user_at_room in UserInRoom_entities]
        self.response.out.write(json.dumps(usernames))

错误是:

Traceback (most recent call last):
  File "C:\Program Files\Google\google_appengine\google\appengine\ext\webapp\__init__.py", line 507, in __call__
    handler.get(*groups)
  File "path\to\chat.py", line 160, in get
AttributeError: 'NoneType' object has no attribute 'append'

这怎么可能？我在调用之前立即设置 UserInRoom_entities = []。还是 None 有问题？

更新:此代码有效:

class UsersInRoom(webapp.RequestHandler):
    def get(self):
        room_id = self.request.get("room_id")
        username = self.request.get("username")
        UserInRoom_entities = UserInRoom.gql("WHERE room = :1", room_id).get()
        if UserInRoom_entities:
            for user_in_room in UserInRoom_entities:
                if user_in_room.name == username:
                    user_in_room.put() # last_modified auto updates to now whenenever user_in_room is saved
        else:
            user_in_room = UserInRoom(room=Key(room_id), name=username)
            user_in_room.put()

            UserInRoom_entities = []
            UserInRoom_entities.append(user_in_room)

        # name is `user_at_room` intead of `user_in_room` to avoid confusion    
        usernames = [user_at_room.name for user_at_room in UserInRoom_entities]
        self.response.out.write(json.dumps(usernames))

class ChatRoom(db.Model):
    name = db.StringProperty()
    last_modified = db.DateTimeProperty(auto_now=True)
    message_contents = db.StringListProperty()
    message_users = db.StringListProperty()

class UserInRoom(db.Model):
    room = db.ReferenceProperty(ChatRoom)
    name = db.StringProperty()
    last_modified = db.DateTimeProperty(auto_now=True)

Answer 1

由于我对问题的评论似乎已经回答了这个问题，我将重复它作为答案，希望获得一些声望点:

UserInRoom 实例是否正确初始化？我不熟悉 GAE 数据模型，但我可以想象 put() 实例将需要设置 room 属性，如果有关系在 UserInRoom 和 Room 之间(假设存在一个 Room 类)。