maybe.py

from contextlib import contextmanager
from models import User


@contextmanager
def maybe(thing, check=lambda thing: thing):
    if check(thing):
        yield thing


# Rather than this
def filter_ads_for_thread(ads, thread):
    blacklisted_ads = set(BlackListedForumsModel.objects.using('default.slave').filter(
        advertisement__in=ads,
        forum=thread.forum,
    ).values_list('advertisement_id', flat=True))
    if blacklisted_ads:
        ads = [ad for ad in ads if ad.id not in blacklisted_ads]

    publisher_info = thread.forum.get_publisher_info()
    if publisher_info:
        for domain in publisher_info.blacklisted_domains:
            ads = [ad for ad in ads if not domain_match(ad.parsed_url, domain)]
    return ads


# You can do this
def filter_ads_for_thread(ads, thread):
    blacklisted_ads = BlackListedForumsModel.objects.using('default.slave').filter(
        advertisement__in=ads,
        forum=thread.forum,
    ).values_list('advertisement_id', flat=True)

    with maybe(blacklisted_ads) as blacklist_result:
        ads = [ad for ad in ads if ad.id not in blacklist_result]

    with maybe(thread.forum.get_publisher_info()) as publisher_info:
        for domain in publisher_info.blacklisted_domains:
            ads = [ad for ad in ads if not domain_match(ad.parsed_url, domain)]

    return ads