Python类变量与状态实例：解耦设计与循环依赖避免

# 初始设计，存在循环依赖问题
class State:
    # 问题所在：StartState 和 EndState 在此处尚未定义
    START: "State" = StartState()
    END: "State" = EndState()

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

class StartState(State):
    # StartState 的具体实现
    pass

class EndState(State):
    # EndState 的具体实现
    pass

# Context 类定义（为完整示例提供）
class Context:
    def __init__(self, just_beginning: bool = False):
        self.just_beginning = just_beginning

# 优化后的状态基类
class State:
    pass

# 全局常量实例，在State类定义后创建
# 按照Python约定，常量名通常使用全大写
START_STATE = State()
END_STATE = State()

# 优化后的Context类，包含获取状态的方法
class Context:
    def __init__(self, just_beginning: bool = False):
        self.just_beginning = just_beginning

    def get_state(self) -> State:
        """
        根据当前上下文的条件返回相应的状态实例。
        """
        if self.just_beginning:
            return START_STATE
        return END_STATE

# State类和全局状态实例保持不变
# class State:
#     pass
# START_STATE = State()
# END_STATE = State()

# 1. 定义状态基类
class State:
    """
    状态基类，可以根据需要添加通用方法或属性。
    """
    def __repr__(self):
        return f"<{self.__class__.__name__} object at {hex(id(self))}>"

# 2. 定义上下文类，负责根据自身条件获取状态
class Context:
    """
    上下文类，持有决定当前状态所需的信息。
    """
    def __init__(self, just_beginning: bool = False):
        self.just_beginning = just_beginning

    def get_state(self) -> State:
        """
        根据上下文的just_beginning属性返回起始或结束状态。
        """
        if self.just_beginning:
            return START_STATE
        return END_STATE

# 3. 在所有相关类定义完成后，创建全局状态实例
# 这些实例作为程序中的“常量”状态对象
START_STATE = State()
END_STATE = State()

# 示例用法
if __name__ == "__main__":
    context_beginning = Context(just_beginning=True)
    current_state_1 = context_beginning.get_state()
    print(f"When just beginning: {current_state_1} (Is it START_STATE? {current_state_1 is START_STATE})")

    context_normal = Context(just_beginning=False)
    current_state_2 = context_normal.get_state()
    print(f"When not beginning: {current_state_2} (Is it END_STATE? {current_state_2 is END_STATE})")

    # 验证状态实例的唯一性
    another_start = context_beginning.get_state()
    print(f"Another call for start state: {another_start} (Is it the same instance? {another_start is START_STATE})")