unit-testing

Unit Testing Standards

Quick Navigation: Level 1: Quick Start (5 min) → Level 2: Implementation (30 min) → Level 3: Mastery (Extended)

Level 1: Quick Start (<2,000 tokens, 5 minutes)

Core Principles

1. Test-Driven Development (TDD): Write tests before implementation (Red-Green-Refactor)
2. Test Pyramid: 70% unit tests, 20% integration, 10% E2E
3. Fast and Isolated: Tests run in milliseconds, no external dependencies
4. Comprehensive Coverage: Aim for 80-90% code coverage minimum
5. Clear and Maintainable: Tests serve as living documentation

Essential Checklist

• TDD workflow: Write failing test → Implement → Refactor
• Coverage targets: 80%+ overall, 95%+ for critical paths
• Naming convention: test_<function>_<scenario>_<expected_result>
• AAA pattern: Arrange, Act, Assert structure in every test
• Mocking: Mock external dependencies (database, APIs, filesystem)
• Fixtures: Reusable test data and setup/teardown
• Parametrized tests: Test multiple inputs efficiently
• CI integration: Tests run automatically on every commit

Quick Example

# pytest unit testing example
import pytest
from datetime import datetime

def calculate_discount(user_age: int, purchase_amount: float) -> float:
    """Calculate discount based on age and purchase amount."""
    if user_age < 18:
        return 0.0
    elif user_age >= 65:
        return purchase_amount * 0.15
    elif purchase_amount >= 100:
        return purchase_amount * 0.10
    return 0.0

# Unit tests following TDD
def test_calculate_discount_no_discount_for_minors():
    """Test that users under 18 receive no discount."""
    # Arrange
    user_age = 16
    purchase_amount = 100.0

    # Act
    discount = calculate_discount(user_age, purchase_amount)

    # Assert
    assert discount == 0.0

def test_calculate_discount_senior_discount():
    """Test that seniors (65+) receive 15% discount."""
    assert calculate_discount(65, 100.0) == 15.0
    assert calculate_discount(70, 200.0) == 30.0

def test_calculate_discount_large_purchase():
    """Test that purchases >= $100 receive 10% discount."""
    assert calculate_discount(30, 100.0) == 10.0
    assert calculate_discount(30, 150.0) == 15.0

@pytest.mark.parametrize("age,amount,expected", [
    (16, 100, 0.0),   # Minor
    (30, 50, 0.0),    # No discount
    (30, 100, 10.0),  # Large purchase
    (65, 50, 7.5),    # Senior
    (70, 200, 30.0),  # Senior large purchase
])
def test_calculate_discount_parametrized(age, amount, expected):
    """Test multiple discount scenarios."""
    assert calculate_discount(age, amount) == expected