import os


class Singleton:
    """
    单例模式
    """
    def __new__(cls, *args, **kwargs):
        if not hasattr(cls, "_instance"):
            cls._instance = super(Singleton, cls).__new__(cls)
        return cls._instance


class Const(Singleton):
    """
    单例常量类
    """
    class ConstError(TypeError):
        pass

    class ConstCaseError(ConstError):
        pass

    def __setattr__(self, name, value):
        if name in self.__dict__:
            raise self.ConstError(f"can't change const {name}")
        self.__dict__[name] = value


const = Const()  # 常量实例