Test Aaa Structure

Instructions

The AAA Pattern

Arrange-Act-Assert (AAA) is the gold standard for structuring tests. It makes tests:

• Easy to read and understand
• Simple to maintain
• Clear about what's being tested

The Three Phases

1. Arrange: Set Up

Purpose: Prepare everything needed for the test.

What to do:

• Create test data
• Set up mock objects
• Initialize dependencies
• Configure system state

Example:

# Arrange
user = User(id="123", name="Alice", role="admin")
db.save(user)
mock_logger = Mock(Logger)

2. Act: Execute

Purpose: Call the thing you're testing.

What to do:

• Call ONE function or method
• Capture the result
• Keep this section minimal (usually 1 line)

Example:

# Act
result = get_user_by_id("123")

Common Mistake: ❌ Calling multiple functions in Act Correct: ✅ One function call - if you need multiple, you need multiple tests

3. Assert: Verify

Purpose: Check that the behavior is correct.

What to do:

• Verify return values
• Check side effects
• Validate state changes
• Confirm error handling

Example:

# Assert
assert result.name == "Alice"
assert result.role == "admin"
assert mock_logger.log.called_once()

Key Rules

Rule 1: One Logical Assertion Per Test

Each test should verify ONE behavior.

✅ Good:

def test_get_user_returns_correct_name():
    user = User(id="1", name="Alice")
    db.save(user)
    result = get_user_by_id("1")
    assert result.name == "Alice"

def test_get_user_returns_correct_role():
    user = User(id="1", role="admin")
    db.save(user)
    result = get_user_by_id("1")
    assert result.role == "admin"

❌ Bad:

def test_get_user():  # What does this test? Unclear!
    user = User(id="1", name="Alice", role="admin")
    db.save(user)
    result = get_user_by_id("1")
    assert result.name == "Alice"  # Multiple unrelated assertions
    assert result.role == "admin"
    assert result.id == "1"

Exception: Multiple assertions are OK if testing a single concept:

def test_user_creation_sets_all_required_fields():
    user = create_user(name="Alice", email="alice@example.com")
    assert user.name == "Alice"  # All part of "creation"
    assert user.email == "alice@example.com"
    assert user.created_at is not None

Rule 2: Descriptive Test Names

Test names should read like documentation.

Pattern: test_{what}_{does}_{when}_{condition}

✅ Good:

• test_get_user_returns_user_when_found
• test_get_user_returns_none_when_not_found
• test_get_user_raises_error_when_database_unavailable

❌ Bad:

• test_get_user (what does it test?)
• test1, test2 (meaningless)
• test_success (success of what?)

Rule 3: Visual Separation

Separate AAA sections for readability.

✅ Option 1: Comments

def test_calculate_total_includes_tax():
    # Arrange
    cart = Cart()
    cart.add_item(Item(price=100))
    
    # Act
    total = cart.calculate_total(tax_rate=0.1)
    
    # Assert
    assert total == 110

✅ Option 2: Blank lines

def test_calculate_total_includes_tax():
    cart = Cart()
    cart.add_item(Item(price=100))
    
    total = cart.calculate_total(tax_rate=0.1)
    
    assert total == 110

❌ Bad: No separation

def test_calculate_total_includes_tax():
    cart = Cart()
    cart.add_item(Item(price=100))
    total = cart.calculate_total(tax_rate=0.1)
    assert total == 110  # Hard to see structure

Common Patterns

Testing Exceptions

def test_divide_raises_error_when_divisor_is_zero():
    # Arrange
    calculator = Calculator()
    
    # Act & Assert (combined for exceptions)
    with pytest.raises(ZeroDivisionError):
        calculator.divide(10, 0)

Testing Side Effects

def test_save_user_writes_to_database():
    # Arrange
    user = User(id="123", name="Alice")
    db = MockDatabase()
    
    # Act
    save_user(user, db)
    
    # Assert
    assert db.write_called_with(user)

Testing Async Functions

async def test_fetch_user_calls_api():
    # Arrange
    mock_client = Mock(ApiClient)
    mock_client.get.return_value = {"name": "Alice"}
    
    # Act
    result = await fetch_user("123", mock_client)
    
    # Assert
    assert result.name == "Alice"
    assert mock_client.get.called_with("/users/123")

Anti-Patterns to Avoid

❌ Testing Implementation Instead of Behavior

Bad:

def test_sort_uses_quicksort():  # Implementation detail!
    result = my_sort([3, 1, 2])
    assert used_quicksort_algorithm(result)

Good:

def test_sort_returns_sorted_list():  # Behavior!
    result = my_sort([3, 1, 2])
    assert result == [1, 2, 3]

❌ Too Much Setup

Bad:

def test_user_login():
    # Arrange (50 lines of setup!)
    db = Database()
    db.connect()
    db.create_schema()
    users_table = db.create_table("users")
    # ... 40 more lines ...

Good:

def test_user_login():
    # Arrange (use fixtures/factories)
    user = create_test_user()  # Helper function
    # Act & Assert...

❌ Multiple Acts

Bad:

def test_user_workflow():
    user = create_user()  # Act 1
    login(user)  # Act 2  
    result = get_profile(user)  # Act 3
    assert result.name == "Alice"

Good: Split into 3 tests (one per Act)

Quick Reference

Arrange → Set up
Act     → Execute
Assert  → Verify

Remember:

• ✅ One behavior per test
• ✅ Descriptive test names
• ✅ Visual AAA separation
• ❌ No implementation testing
• ❌ No multiple Acts