Check if the content of the given document is present
        in the cache

        If the max_age attribute of this Cache is set to a nonzero value,
        entries older than the value of max_age in seconds will be ignored

        :type doc_url: String or Document
        :param doc_url: the URL of the document, or a Document object

        :rtype: Boolean
        :returns: True if the data is present, False otherwise

def has_document(self, doc_url):
        """ Check if the content of the given document is present
        in the cache

        If the max_age attribute of this Cache is set to a nonzero value,
        entries older than the value of max_age in seconds will be ignored

        :type doc_url: String or Document
        :param doc_url: the URL of the document, or a Document object

        :rtype: Boolean
        :returns: True if the data is present, False otherwise


        """
        c = self.conn.cursor()
        c.execute("SELECT * FROM documents WHERE url=?", (str(doc_url),))
        row = c.fetchone()
        c.close()
        return self.__exists_row_not_too_old(row)