class builder:
    """
    This is class responsible for building new items for database.
    """

    def __init__(self, target: object = None) -> None:
        """ 
        This create new builder, cleaning it by default.

        Parameters:
            target (object<target_type>): Object to work on
        """

        self.clear(target)

    @property
    def _target(self) -> object:
        """ Target to work on for child class. """

        return self.__target

    @property
    def result(self) -> object:
        """
        This return created object, or raise exception if it is not ready.

        Returns:
            (object<target_type>): Ready created object
        """

        if not self.__target.ready:
            raise TypeError("Can not read not ready result.")

        return self.__target

    def clear(self, target: object = None) -> None:
        """
        This drop previous object from builded, and set new one. When target
        is not set, it create empty object to work on.

        Parameters:
            target (object<target_type> | None): Target to work on or None
        """

        if target is not None:
            self.__target = target
            return

        self.__target = self.__target_type()

    def __init_subclass__(cls, target_type: type, **kwargs) -> None:
        """
        This create new class. It require target_type, which is class of new
        objects. It must have ready property, to check when new object is 
        ready to load.

        Parameters:
            target_type(type): Type of target objects
            **kwargs: Rest of parameters
        """

        super().__init_subclass__(**kwargs)

        if not type(target_type.ready) is property:
            raise Exception("Builder can build only database items.")

        cls.__target_type = target_type