Engineering Craft › Testing

Arrange, Act, Assert

The standard three-part structure of a test.

Also known as: AAA, AAA pattern, given when then

Arrange, Act, Assert is the standard three-part structure for a test. Keeping to it makes tests easy to read.

1. Arrange: set up what the test needs (objects, data, fakes).
2. Act: do the one thing you’re testing.
3. Assert: check the result.

def test_discount_applies_to_orders_over_100():
    # Arrange
    order = Order(items=[Item(price=120)])
    coupon = Coupon(percent=10)

    # Act
    total = order.apply(coupon).total()

    # Assert
    assert total == 108

Blank lines between the sections (or comments, if the team likes them) make the structure obvious.

Why it helps

• Readable: anyone can see what’s being set up, what’s tested and what’s expected.
• Focused: one Act per test, so a failure points at one behavior.
• Easy to spot smells: a huge Arrange suggests the code has too many dependencies. Several Acts mean the test is doing too much.

Habits

• One behavior per test, with a name that says it: test_expired_coupon_is_rejected.
• Put only relevant details in Arrange. Use helpers or builders for the rest (test fixtures).
• Don’t put logic in tests (loops, conditionals). If a test needs an if, it’s hard to trust.
• Assert on results, not on how the code got there.
• Keep Act simple, usually one call.

Given / When / Then is the same shape in behavior-driven style (given-when-then, BDD).