Construct a filename for a database.

        Parameters:
        batchno -- batch number for the rotated database.

        Returns the constructed path as a string.

def _construct_filename(self, batchno):
        """Construct a filename for a database.

        Parameters:
        batchno -- batch number for the rotated database.

        Returns the constructed path as a string.

        """
        return os.path.join(self.dirpath,
                            "{0}.{1}".format(self.prefix, batchno))