Django optimize for loop in method

Question

I wrote the method to check one attribute and return False if there is an artist with selling=False, and True otherwise.

def check_selling(track, excludes):
    """
    Returns True if all track's artists are allowed for selling. False otherwise
    """
    for artist in track.artists.all():
            if not artist.selling:
                excludes.append(track.pk)
                return False
    return True

How can i minimize this?

I tried:

def check_selling(track, excludes):
    res = excludes.append(track.pk) if not [artist.selling for artist in track.artists.all()] else True
    return res or False

But [False] or [True] resulting in list comprehension [artist.selling for artist in track.artists.all()] always gives True...

What is "artist.selling" ? can it be null or empty string?

Leandro
– Leandro

2013-06-17 18:09:25 +00:00
Commented Jun 17, 2013 at 18:09 — Leandro
– Leandro, Commented Jun 17, 2013 at 18:09
boolean field, default=true

Feanor
– Feanor

2013-06-17 18:26:45 +00:00
Commented Jun 17, 2013 at 18:26 — Feanor
– Feanor, Commented Jun 17, 2013 at 18:26

Leandro · Accepted Answer · 2013-06-17 18:19:54Z

2

Make Queries! See Here

def check_selling(track, excludes):
    """
    Returns True if all track's artists are allowed for selling. False otherwise
    """
    if track.artists.filter(selling=False).exists(): #If "selling" is a boolean
        excludes.append(track)
        return False
    return True

Just one query

edited Jun 17, 2013 at 18:19

answered Jun 17, 2013 at 18:12

Leandro

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Comments

000 · Accepted Answer · 2013-06-17 18:02:45Z

2

http://docs.python.org/2/library/functions.html#all

New in version 2.5.

def check_selling(track, excludes):
    """
    Returns True if all track's artists are allowed for selling. False otherwise
    """
    if all( [artist.selling for artist in track.artists.all()] ):
        return True
    excludes.append(track.pk)
    return False

answered Jun 17, 2013 at 18:02

000

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2 Comments

000 Over a year ago

I do recommend you look at Sylvain Leroux's answer.

Leandro Over a year ago

Yes, You are making one query for artist in track

Sylvain Leroux · Accepted Answer · 2013-06-17 18:09:40Z

2

premature optimization is the root of all evil -- Donald Knuth

... but, it is no question of optimization here. But of doing right.

It is highly inefficient to retrieve all records just to count those having a given attribute. You could do much better at SQL level:

SELECT COUNT(*) FROM artist WHERE SELLING != false

That statement will directly return the number of artist not selling. Not only this will reduce traffic between the RDBMS and your application -- but in some cases, the RDBMS will be able to "optimize" this statement, either by using an index (if you have one on "selling") and/or its cache. Depending your BD backend, the syntax may vary. The good new is that Django has support for such query using count(). Something like that:

artist.objects.filter(selling!=false).count()

answered Jun 17, 2013 at 18:09

Sylvain Leroux

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1 Comment

Feanor Over a year ago

Thanks, but the answer with the use of related manager is more appreciated. But i see your way, it's what i need.

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