Rivet

title: Use Comprehensions Over for+append Loops impact: LOW impactDescription: more concise, often faster, and idiomatic Python tags: simplify, comprehensions, idioms

Use Comprehensions Over for+append Loops

Comprehensions express "build a collection from an iterable" in one line. C-style loops with append() have more variables and more places for off-by-one and wrong-list bugs. Reach for a comprehension by default.

Incorrect (imperative loop + append):

def active_usernames(users: list[User]) -> list[str]:
    result = []
    for user in users:
        if user.is_active:
            result.append(user.name)
    return result

Correct (list, dict, set, and generator forms):

def active_usernames(users: list[User]) -> list[str]:
    return [user.name for user in users if user.is_active]

name_to_id = {user.name: user.id for user in users}
unique_tags = {tag for post in posts for tag in post.tags}
total = sum(item.price for item in items)   # generator, no intermediate list

When to break a comprehension into a loop: when the expression stops reading like English — multi-step logic, side effects, or nested conditionals with intermediate variables are signs the comprehension has outgrown one line. For boolean reductions, prefer any(u.is_admin for u in users) over any([...]) — the generator short-circuits and avoids materializing the list.