python - 使用 Python 进行延迟加载的模式

Question

我有字典形式的数据(通过 PyMongo 来自 MongoDB 数据库)，例如:

car = {_id: "1", "color": "silver", "steering_wheel":"2"}

其中“steering_wheel”的值是我数据库中另一个文档的 ID，它表示类 SteeringWheel 的一个实例。从数据库加载到 Python 将导致:

steering_wheel = {_id: "2", "cover": "plastic"}

为了处理数据，我使用 Python 类。现在，我的问题是关于延迟加载的。我可以想到两种方法:

1) 保留引用的 id 并创建另一个仅运行时的属性以直接访问引用的对象:

class Car(object):
    def __init__(self, _id, color, steering_wheel_ref_id, session):
        self._id = _id
        self.color = color
        self.steering_wheel_ref_id = steering_wheel_ref_id
        self.session = session

    @property
    def steering_wheel(self):
        try:
            return self.steering_wheel
        except AttributeError:
            # Get the referecend object from the session 
            self.steering_wheel = self.session.resolve_item_refs(self.steering_wheel_ref_id)
            return self.steering_wheel

2) 另一种选择是执行类型检查:

class Car(object):
    def __init__(self, _id, color, steering_wheel, session):
        self._id = _id
        self.color = color
        self.steering_wheel = steering_wheel
        self.session = session

    @property
    def steering_wheel(self):
        if isinstance(self.steering_wheel, SteeringWheel):
            return self.steering_wheel
        else:
            # Get the referecend object from the session 
            self.steering_wheel = self.session.resolve_item_refs(self.steering_wheel_ref_id)
            return self.steering_wheel

你更喜欢哪种方式？或者是否有更好的方法/最佳实践来解决通过 ID 访问引用？

Answer 1

这个怎么样？

class Car(object):
    def __init__(self, _id, color, steering_wheel_ref_id, session):
        self._id = _id
        self.color = color
        self.steering_wheel_ref_id = steering_wheel_ref_id
        self.session = session
        self._steering_wheel = None

    @property
    def steering_wheel(self):
        if self._steering_wheel is None:
            # Get the referecend object from the session 
            self._steering_wheel = self.session.resolve_item_refs(self.steering_wheel_ref_id)
        return self._steering_wheel