Read more data from the file.

def __retrieve_data(self):
        """Read more data from the file."""

        if self.__eof is True:
            return b''

        logging.debug("Reading another block.")        
        block = self.read(self.__block_size)
        if block == b'':
            self.__log.debug("We've encountered the EOF.")
            self.__eof = True

        return block