Order by before where (ARCH2076)

Key python.performance.orderby-before-where · Severity warning · Category performance

Sorting before filtering wastes work by ordering elements that will be discarded. Filter first, then sort the smaller remaining set.

Incorrect

def top_active(users):
    ordered = sorted(users, key=lambda u: u.score, reverse=True)
    return [u for u in ordered if u.active][:10]

Correct

def top_active(users):
    active = [u for u in users if u.active]
    return sorted(active, key=lambda u: u.score, reverse=True)[:10]

Fix

Filter first and apply the sort only to the smaller remaining set.