Dataflow/apache beam - how to access current filename when passing in pattern?

I have seen this question answered before on stack overflow (https://stackoverflow.com/questions/29983621/how-to-get-filename-when-using-file-pattern-match-in-google-cloud-dataflow), but not since apache beam has added splittable dofn functionality for python. How would I access the filename of the current file being processed when passing in a file pattern to a gcs bucket?

I want to pass the filename into my transform function:

with beam.Pipeline(options=pipeline_options) as p:                              
    lines = p | ReadFromText('gs://url to file')                                        


    data = (                                                                    
        lines                                                                   
        | 'Jsonify' >> beam.Map(jsonify)                                        
        | 'Unnest' >> beam.FlatMap(unnest)                                      
        | 'Write to BQ' >> beam.io.Write(beam.io.BigQuerySink(                  
            'project_id:dataset_id.table_name', schema=schema,                     
            create_disposition=beam.io.BigQueryDisposition.CREATE_IF_NEEDED,    
            write_disposition=beam.io.BigQueryDisposition.WRITE_APPEND)       
        )                                                   

and possibly have to use the filename to use side inputs and bring in some more data based on the filename.

from Dataflow/apache beam - how to access current filename when passing in pattern?