def _query(self, data_category, resource_category, field, request, step, isotime=None):
        """
        Request and return data from DataPoint RESTful API.
        """
        rest_url = "/".join([HOST, data_category, resource_category, field, DATA_TYPE, request])
        query_string = "?" + "&".join(["res=" + step, "time=" + isotime if isotime is not None else "", "key=" + self.key])
        url = rest_url + query_string
        page = url_lib.urlopen(url)
        pg = page.read()
        return pg