How to use the pylsl.cf_string function in pylsl

for k, stream in enumerate(self.streams):
                n = stream.name()
                stream_params = copy.deepcopy(self.def_stream_parms)
                stream_params['metadata'].update({
                    "name": n,
                    "ch_count": stream.channel_count(),
                    "ch_format": stream.channel_format(),
                    "srate": stream.nominal_srate()
                })
                # ch = stream.desc().child("channels").child("channel")
                # for ch_ix in range(stream.channel_count()):
                #     print("  " + ch.child_value("label"))
                #     ch = ch.next_sibling()

                stream_params['inlet'] = pylsl.StreamInlet(stream)
                stream_params['is_marker'] = stream.channel_format() in ["String", pylsl.cf_string]\
                                             and stream.nominal_srate() == pylsl.IRREGULAR_RATE
                if not stream_params['is_marker']:
                    if self.sig_strm_idx < 0:
                        self.sig_strm_idx = k
                    srate = stream.nominal_srate()
                    stream_params['downSampling'] = srate > 1000
                    stream_params['chunkSize'] = round(srate / self.chunksPerScreen * self.seconds_per_screen)
                    if stream_params['downSampling']:
                        stream_params['downSamplingFactor'] = round(srate / 1000)
                        n_buff = round(stream_params['chunkSize'] / stream_params['downSamplingFactor'])
                        stream_params['downSamplingBuffer'] = [[0] * int(stream.channel_count())] * n_buff
                self.stream_params.append(stream_params)

            self.updateStreamNames.emit([_['metadata'] for _ in self.stream_params], self.sig_strm_idx)
            self.start()

def _create_python_repr_lsl_outlet(self, name):
        """Create 1 channel python representation outlet"""
        stream_info = lsl.StreamInfo(
            name=name,
            type='Pupil Capture',
            channel_count=1,
            channel_format=lsl.cf_string,
            source_id='%s @ %s' % (name, self.g_pool.ipc_sub_url))
        self._append_channel_info(stream_info, ("Python Representation",))
        self._append_acquisition_info(stream_info)
        return lsl.StreamOutlet(stream_info)