Get attribute for Beautifulsoup object
        :param soup: Beautifulsoup object
        :param key: attribute key
        :param unknown: attribute key not exists value(default:None)
        :return: attribute value

def get_attribute(cls, soup, key, unknown=None):
        """
        Get attribute for Beautifulsoup object
        :param soup: Beautifulsoup object
        :param key: attribute key
        :param unknown: attribute key not exists value(default:None)
        :return: attribute value
        """
        if key in soup.attrs:
            return soup.get(key)
        return unknown