Use this function in emit data into the store.

        :param data_frame: DataFrame to be recorded.

def emit(self, data_frame):
        """Use this function in emit data into the store.

        :param data_frame: DataFrame to be recorded.
        """
        if self.result is not None:
            raise MultipleEmitsError()
        data_frame.columns = [self.prefix + '__' + c
                              for c in data_frame.columns]
        self.result = data_frame