A recommendation job stores each user's favourite genre as {user: genre}. A marketing email needs it the other way round: for each genre, who likes it.
Write a function fans_by_genre(favourites) that returns a dictionary mapping each genre to a sorted list of users, with the genres in alphabetical order too.
How it's checked
Your code runs, then the check below runs and its printed output is compared with ours.
fans_by_genre({"u3": "jazz", "u1": "rock", "u2": "jazz"}) returns {"jazz": ["u2", "u3"], "rock": ["u1"]}Hints, the later steps and the answer come with Pro.
def fans_by_genre(favourites):
...