code: def __get_indexer(in_fns, selected_type=None):
    
    indexer = None
    if selected_type is not None:
      indexer = get_indexer_by_filetype(selected_type)
    else:
      if len(in_fns) == 0:
        raise IndexError("reading from stdin, unable to guess input file " +
                         "type, use -t option to set manually.\n")
      else:
        extension = set([os.path.splitext(f)[1] for f in in_fns])
        assert(len(extension) >= 1)
        if len(extension) > 1:
          raise IndexError("more than one file extension present, unable " +
                           "to get input type, use -t option to set manually.\n")
        else:
          indexer = get_indexer_by_file_extension(list(extension)[0])
    assert(indexer is not None)
    return indexer
docs: Determine which indexer to use based on input files and type option.