code: def getDatastream(self, pid, dsID, asOfDateTime=None, validateChecksum=False):
          
          # /objects/{pid}/datastreams/{dsID} ? [asOfDateTime] [format] [validateChecksum]
          http_args = {}
          if validateChecksum:
              # fedora only responds to lower-case validateChecksum option
              http_args['validateChecksum'] = str(validateChecksum).lower()
          if asOfDateTime:
              http_args['asOfDateTime'] = datetime_to_fedoratime(asOfDateTime)
          http_args.update(self.format_xml)
          uri = 'objects/%(pid)s/datastreams/%(dsid)s' % {'pid': pid, 'dsid': dsID}
          return self.get(uri, params=http_args)
docs: Get information about a single datastream on a Fedora object; optionally,
          get information for the version of the datastream as of a particular date time.
  
          :param pid: object pid
          :param dsID: datastream id
          :param asOfDateTime: optional datetime; ``must`` be a non-naive datetime
              so it can be converted to a date-time format Fedora can understand
          :param validateChecksum: boolean; if True, request Fedora to recalculate
              and verify the stored checksum against actual data
          :rtype: :class:`requests.models.Response`