'A tuple of allowed genre sources. May contain \'track\',
\'album\', or \'artist.\''

@property
def sources(self):

    source = self.config['source'].as_choice(('track', 'album', 'artist'))
    if (source == 'track'):
        return ('track', 'album', 'artist')
    elif (source == 'album'):
        return ('album', 'artist')
    elif (source == 'artist'):
        return ('artist',)
