def arg_dict(self):
        """Get dictionary representation of argument arrrays.

        Returns
        -------
        arg_dict : dict of str to NDArray
            The dictionary that maps the names of arguments to NDArrays.

        Raises
        ------
        ValueError : if there are duplicated names in the arguments.
        """
        if self._arg_dict is None:
            self._arg_dict = Executor._get_dict(
                self._symbol.list_arguments(), self.arg_arrays)
        return self._arg_dict