Python中如何处理类变量为自身子类的引用问题？

你遇到的问题本质是Python类定义的执行顺序限制：定义State类时，StartState和EndState还未被创建，直接赋值会触发NameError；但先定义子类又会因父类State未定义导致继承报错。下面提供几种可行的解决思路：

方法1：子类定义后再赋值父类类变量 #

先定义父类，再定义子类，最后手动给父类的类变量赋值，完全贴合Python执行逻辑：

class State:
    START: "State"  # 用字符串类型注解暂存类型提示
    END: "State"

    @classmethod
    def get_current(cls, context: Context) -> "State":
        if context.just_beginning:
            return cls.START
        return cls.END

class StartState(State):
    ...

class EndState(State):
    ...

# 子类定义完成后补全赋值
State.START = StartState()
State.END = EndState()

方法2：类属性延迟加载（@classmethod + property） #

把START和END改成类属性形式，第一次访问时才初始化子类实例，适合懒加载场景：

class State:
    _START: "State" = None
    _END: "State" = None

    @classmethod
    @property
    def START(cls) -> "State":
        if cls._START is None:
            cls._START = StartState()
        return cls._START

    @classmethod
    @property
    def END(cls) -> "State":
        if cls._END is None:
            cls._END = EndState()
        return cls._END

    @classmethod
    def get_current(cls, context: Context) -> "State":
        if context.just_beginning:
            return cls.START
        return cls.END

class StartState(State):
    ...

class EndState(State):
    ...

方法3：利用__init_subclass__自动注册子类 #

通过Python的__init_subclass__钩子，让子类定义时自动注册到父类的类变量中，适合多子类动态注册场景：

class State:
    START: "State"
    END: "State"

    @classmethod
    def get_current(cls, context: Context) -> "State":
        if context.just_beginning:
            return cls.START
        return cls.END

    @classmethod
    def __init_subclass__(cls, state_type=None, **kwargs):
        super().__init_subclass__(**kwargs)
        if state_type == "start":
            State.START = cls()
        elif state_type == "end":
            State.END = cls()

# 定义子类时指定类型标识完成自动注册
class StartState(State, state_type="start"):
    ...

class EndState(State, state_type="end"):
    ...

内容的提问来源于stack exchange，提问作者Chetchaiyan