Write just one file per window with WriteToFiles transform

input = (p
                 | 'ReadData' >> beam.io.ReadFromPubSub(topic=known_args.input_topic).with_output_types(bytes)
                 | "Decode" >> beam.Map(lambda x: x.decode('utf-8'))
                 | 'Parse' >> beam.Map(parse_json)
                 | 'Data w' >> beam.WindowInto(
                    FixedWindows(60),
                    trigger=trigger.AfterWatermark(),
                    accumulation_mode=AccumulationMode.DISCARDING
                )
                 | 'Group elements into windows' >> beam.Reshuffle()
                 )

        event_data = (input
             | 'Filter events' >> beam.Filter(lambda x: x['t'] == 'event')
             | 'Encode ' >> beam.Map(lambda x: json.dumps(x))
             | 'Write to file's' >> fileio.WriteToFiles(
                    path='gs://some/gcs/bucket/',
                    file_naming=fileio.default_file_naming(
                        prefix='events',
                        suffix='.txt'
                    ),
                    shards=1
                )
        )