code: def ref_file_from_bam(bam_file, data):
      
      new_ref = os.path.join(utils.safe_makedir(os.path.join(dd.get_work_dir(data), "inputs", "ref")),
                             "%s-subset.fa" % dd.get_genome_build(data))
      if not utils.file_exists(new_ref):
          with file_transaction(data, new_ref) as tx_out_file:
              contig_file = "%s-contigs.txt" % utils.splitext_plus(new_ref)[0]
              with open(contig_file, "w") as out_handle:
                  for contig in [x.contig for x in idxstats(bam_file, data) if x.contig != "*"]:
                      out_handle.write("%s\n" % contig)
              cmd = "seqtk subseq -l 100 %s %s > %s" % (dd.get_ref_file(data), contig_file, tx_out_file)
              do.run(cmd, "Subset %s to BAM file contigs" % dd.get_genome_build(data))
      ref.fasta_idx(new_ref, data["config"])
      runner = broad.runner_from_path("picard", data["config"])
      runner.run_fn("picard_index_ref", new_ref)
      return {"base": new_ref}
docs: Subset a fasta input file to only a fraction of input contigs.