What makes a good unit test?

Summary

A good unit test is fast, deterministic, isolated, and readable; it tests one behavior through the public interface, has a descriptive name, follows Arrange-Act-Assert, and fails for exactly one reason with a clear message.

Longer Answer

Avoid testing implementation details (private methods, exact call sequences) so refactoring doesn't break tests. Prefer real collaborators when they're cheap and deterministic; mock only boundaries such as networks, clocks and randomness.

Code Example

def test_discount_applies_to_orders_over_100():
    # Arrange
    order = Order(items=[Item(price=120)])
    # Act
    total = apply_discount(order, percent=10)
    # Assert
    assert total == 108

Key Points

• Fast and deterministic
• Tests behavior, not implementation
• Arrange-Act-Assert
• Descriptive names