Question

I’m trying to refactor my little voting functions. I’ve got two of them which do almost exactly the same thing, only to different model fields. Voters can add stars to a song if they like it, and they can add flags to a song if they dislike it. The voters are then associated with the song via a many2many relationship, so it can be determined if they already voted on that song. If so, a star (or a flag) is removed from the song and the relationship between voter and song is removed also.

Here is the function that adds or removes stars:

# models.py
class UserProfile(models.Model):
    user = models.ForeignKey(User, unique = True, related_name = 'profile')
    liked_songs = models.ManyToManyField('Song', blank = True, null = True)
    disliked_songs = models.ManyToManyField('Song', blank = True, null = True)

class Song(models.Model):
    song_id = models.CharField(max_length = 36, primary_key = True)
    last_accessed = models.DateTimeField()
    stars = models.IntegerField(max_length = 5, default = 0)
    flags = models.IntegerField(max_length = 5, default = 0)

# views.py
def vote(request, song_id, task):  # 'task' should be either 'stars' or 'flags'
    song = Song.objects.get(song_id = song_id)
    voter = request.user.get_profile()

    # Voter does not already like the song
    if not voter.liked_songs.filter(song_id = song.song_id):  # (*)
        try:
            # Increase vote, set 'last_accessed', add Song to UserProfile.liked_songs
            Song.objects.filter(song_id = song_id).update(
                stars = F('stars') + 1,  # (*)
                last_accessed = datetime.datetime.now()
            )
            voter.liked_songs.add(song)  # (*)
        except:
            raise
    else:
        try:
            # Decrease vote, set 'last_accessed', remove Song from UserProfile.liked_songs
            Song.objects.filter(song_id = song_id).update(
                stars = F('stars') - 1,  # (*)
                last_accessed = datetime.datetime.now()
            )
            voter.liked_songs.remove(song)  # (*)
        except:
            raise
    return "Done."

The function that adds or removes flags is exactly the same, except for the marked (*) lines:

if not voter.disliked_songs.filter(song_id = song.song_id)
flags = F('flags') + 1,
flags = F('flags') - 1,
voter.disliked_songs.add(song)
voter.disliked_songs.remove(song)

Now here’s my first question: What do I have to do to use only one function for liking and disliking a song? I have already introduced the argument task but I can’t seem to write task = F('task') + 1,, because there is no field name ‘task’ in the Song model.

Bonus question: The only difference between stars = F('stars') + 1, and stars = F('stars') - 1, is the operator (+ or -). Is there a way to have an operator in a variable, i.e. task = F('task') myoperator 1,?

Answer 1

Answer for Bonus question: Why dont you have a function that accepts the operator as argument and evaluates the expression based on argument?, like the following example:

>>> def f(op):
...  return (eval("3"+op+"2"))
>>> f("+")
5
>>> f("-")
1