def get_tags(tracks):
    """
        Returns a dict where:
            key: tag_id
            value: list of track ids
    """
    tags = {}
    for _,row in tracks.iterrows():
        t = list(map(lambda x : x.strip(), row['tags'][1:-1].split(',')))
        for x in t:
            if x != '':
                if x in tags:
                    tags[x].append(row['track_id'])
                else:
                    tags[x] = [row['track_id']]
    return tags