Testing Best Practices: pytest and TDD

TIP

Learning Objectives: After this lesson, you'll master pytest with fixtures, parametrization, and mocking, understand test-driven development, achieve meaningful coverage, and write maintainable tests.

Why Testing Matters

Tests are insurance for your code—they catch bugs before users do and give you confidence to refactor.

The Testing Pyramid

pytest Fundamentals

Basic Test Structure

Test Organization

Fixtures: Setup and Teardown

Fixtures provide reusable test setup and cleanup.

Basic Fixtures

Fixture Scopes

Parametrization: Test Multiple Cases

Mocking: Isolate Units

Patching

Test-Driven Development (TDD)

Testing Async Code

Coverage and Quality

Best Practices

Practice Exercises

Exercise 1: Test a Shopping Cart

Key Takeaways

Testing Checklist

• Unit tests for business logic
• Edge cases and error handling
• Integration tests for components
• Mocks for external dependencies
• Async tests with pytest-asyncio
• Coverage > 80% for critical code
• Fast test suite (< 1 minute)
• CI pipeline runs all tests

Next Steps

In the next lesson, we'll explore Design Patterns in Python—implement classic patterns like Factory, Strategy, Observer, and Dependency Injection the Pythonic way.

Ready to write elegant code? Design patterns await!

Further Reading

Official Docs

• pytest Documentation — the test framework nearly everyone uses. Start with Getting Started.
• pytest — Fixtures — the killer feature. Covers dependency-injection, scopes, and the yield fixture pattern.
• pytest — Parametrize — run the same test against many inputs.
• Python — unittest.mock — official mocking. Or use pytest-mock for better ergonomics.

Tutorials & Talks

• Real Python — Getting Started With Testing in Python — the canonical primer.
• Brian Okken — Test & Code podcast — author of Python Testing with pytest. Every episode is gold.

Modern Testing Stack

• pytest-xdist — run your test suite in parallel on all CPU cores.
• pytest-mock — friendlier API for unittest.mock.
• pytest-cov — coverage plugin.
• hypothesis — property-based testing. Finds bugs that example-based tests miss. Life-changing once you adopt it.
• syrupy — modern snapshot testing.
• pytest-asyncio — test async functions.

Mutation & Property-Based Testing

• mutmut — mutation testing. Shows which parts of your code are actually covered by assertions, not just "executed."
• Hypothesis — Quick Start Guide — the 10-minute tour.

Books

• Book: Python Testing with pytest (2nd ed., 2022) — Brian Okken. Definitive.
• Book: Test-Driven Development with Python — Harry Percival (free online). TDD applied to a real Django app.