Returned item should have specific interface. See module docstring.

def get_item(self, url):
        """
        Returned item should have specific interface. See module docstring.
        """

        _hash = self.build_hash(url)
        self.cursor.execute('BEGIN')
        sql = '''
              SELECT data
              FROM cache
              WHERE id = %s
          '''
        self.cursor.execute(sql, (_hash,))
        row = self.cursor.fetchone()
        self.cursor.execute('COMMIT')
        if row:
            data = row[0]
            return self.unpack_database_value(data)
        else:
            return None