def get_artists(tracks):
    """
        Returns a dict where:
            key: artist_id
            value: list of track ids
    """
    artists = {}
    for _,row in tracks.iterrows():
        artist_id = row['artist_id']
        if artist_id in artists:
            artists[artist_id].append(row['track_id'])
        else:
            artists[artist_id] = [row['track_id']]
    return artists