docs: 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