Clean Tests

# Bad - only tests happy path
def test_divide():
    assert divide(10, 2) == 5

# Good - tests edge cases too
def test_divide_normal():
    assert divide(10, 2) == 5

def test_divide_by_zero():
    with pytest.raises(ZeroDivisionError):
        divide(10, 0)

def test_divide_negative():
    assert divide(-10, 2) == -5

# Run with coverage
pytest --cov=myproject --cov-report=term-missing

# Aim for meaningful coverage, not 100%

# Worth having - documents expected behavior
def test_user_default_role():
    user = User(name="Alice")
    assert user.role == "member"

# Bad - hiding a problem
@pytest.mark.skip(reason="flaky, fix later")
def test_async_operation():
    ...

# Good - either fix it or document why it's skipped
@pytest.mark.skip(reason="Requires Redis, see CONTRIBUTING.md for setup")
def test_cache_invalidation():
    ...

def test_pagination_boundaries():
    items = list(range(100))
    
    # First page
    assert paginate(items, page=1, size=10) == items[0:10]
    
    # Last page
    assert paginate(items, page=10, size=10) == items[90:100]
    
    # Beyond last page
    assert paginate(items, page=11, size=10) == []
    
    # Page zero (invalid)
    with pytest.raises(ValueError):
        paginate(items, page=0, size=10)
    
    # Empty list
    assert paginate([], page=1, size=10) == []

# Found bug: off-by-one in date calculation
# Now test ALL date boundaries
def test_month_boundaries():
    assert last_day_of_month(2024, 1) == 31  # January
    assert last_day_of_month(2024, 2) == 29  # Leap year February
    assert last_day_of_month(2023, 2) == 28  # Non-leap February
    assert last_day_of_month(2024, 4) == 30  # 30-day month
    assert last_day_of_month(2024, 12) == 31 # December

# If all async tests fail intermittently,
# the problem isn't the tests—it's the async handling

# If you can't easily test a function, it probably does too much
# Refactor for testability

# Bad - hits real database
def test_user_creation():
    db = connect_to_database()  # Slow!
    user = db.create_user("Alice")
    assert user.name == "Alice"

# Good - uses mock or in-memory
def test_user_creation():
    db = InMemoryDatabase()
    user = db.create_user("Alice")
    assert user.name == "Alice"

# Bad - testing multiple things
def test_user():
    user = User("Alice", "alice@example.com")
    assert user.name == "Alice"
    assert user.email == "alice@example.com"
    assert user.is_valid()
    user.activate()
    assert user.is_active

# Good - one concept each
def test_user_stores_name():
    user = User("Alice", "alice@example.com")
    assert user.name == "Alice"

def test_user_stores_email():
    user = User("Alice", "alice@example.com")
    assert user.email == "alice@example.com"

def test_new_user_is_valid():
    user = User("Alice", "alice@example.com")
    assert user.is_valid()

def test_user_can_be_activated():
    user = User("Alice", "alice@example.com")
    user.activate()
    assert user.is_active