code: def make_gtf_url(ensembl_release, species, server=ENSEMBL_FTP_SERVER):
      
      ensembl_release, species, _ = \
          normalize_release_properties(ensembl_release, species)
      subdir = _species_subdir(
          ensembl_release,
          species=species,
          filetype="gtf",
          server=server)
      url_subdir = urllib_parse.urljoin(server, subdir)
      filename = make_gtf_filename(
          ensembl_release=ensembl_release,
          species=species)
      return join(url_subdir, filename)
docs: Returns a URL and a filename, which can be joined together.