def _get_tag_map(self):

        tag_map = []

        # ignore files that do not match FILE_TYPE
        for position, filename in enumerate(self.files_to_tag):
            logger.debug("track position: %d" % position)
            # add the found files to the tag_map list
            logger.debug("mapping file %s --to--> %s - %s" % (filename,
                         self.album.tracks[position].artist,
                         self.album.tracks[position].title))
            pos = position + 1
            track = self.album.tracks[position]
            track.orig_file = filename
            fileext = os.path.splitext(filename)[1]

            # special handling for Various Artists discs
            if self.album.artist == "Various":
                newfile = self._value_from_tag(self.va_song_format,
                                           track.tracknumber, position, fileext)
            else:
                newfile = self._value_from_tag(self.song_format,
                                           track.tracknumber, position, fileext)

            track.new_file = self._get_clean_filename(newfile)
            tag_map.append(track)

        return tag_map
matches the old with new via TargetTagMap object.
dataset/ETHPy150Open jesseward/discogstagger/discogstagger/taggerutils.py/TaggerUtils._get_tag_map