code: def construct_blastn_cmdline(
      fname1, fname2, outdir, blastn_exe=pyani_config.BLASTN_DEFAULT
  ):
      
      fstem1 = os.path.splitext(os.path.split(fname1)[-1])[0]
      fstem2 = os.path.splitext(os.path.split(fname2)[-1])[0]
      fstem1 = fstem1.replace("-fragments", "")
      prefix = os.path.join(outdir, "%s_vs_%s" % (fstem1, fstem2))
      cmd = (
          "{0} -out {1}.blast_tab -query {2} -db {3} "
          + "-xdrop_gap_final 150 -dust no -evalue 1e-15 "
          + "-max_target_seqs 1 -outfmt '6 qseqid sseqid length mismatch "
          + "pident nident qlen slen qstart qend sstart send positive "
          + "ppos gaps' -task blastn"
      )
      return cmd.format(blastn_exe, prefix, fname1, fname2)
docs: Returns a single blastn command.
  
      - filename - input filename
      - blastn_exe - path to BLASTN executable