name: python-testing description: Use when implementing new Python code (follow TDD), designing test suites, reviewing test coverage, setting up pytest infrastructure, writing fixtures, mocking dependencies, or performing parametrized testing

Python Testing Patterns

Comprehensive testing strategies for Python applications using pytest, TDD methodology, and best practices.

When to Use

• Writing new Python code (follow TDD: red, green, refactor)
• Designing test suites for Python projects
• Reviewing Python test coverage
• Setting up testing infrastructure
• Need guidance on fixtures, mocking, or parametrization

TDD Cycle

Always follow the Red → Green → Refactor cycle:

1. RED: Write a failing test for the desired behavior
2. GREEN: Write minimal code to make the test pass
3. REFACTOR: Improve code while keeping tests green

Quick Start

import pytest

# Basic test
def test_addition():
    assert 2 + 2 == 4

# Test with fixture
@pytest.fixture
def sample_data():
    return {"name": "Alice", "age": 30}

def test_sample_data(sample_data):
    assert sample_data["name"] == "Alice"

# Parametrized test
@pytest.mark.parametrize("input,expected", [
    ("hello", "HELLO"),
    ("world", "WORLD"),
])
def test_uppercase(input, expected):
    assert input.upper() == expected

# Exception testing
def test_divide_by_zero():
    with pytest.raises(ZeroDivisionError):
        _ = 1 / 0

Core Topics

Fixtures

Setup and teardown for tests, sharing data, and scopes (function, module, session). Details: references/fixtures.md

Parametrization

Running tests with multiple inputs using @pytest.mark.parametrize. Details: references/parametrization.md

Mocking and Patching

Using unittest.mock to mock external dependencies and API calls. Details: references/mocking.md

Markers and Test Selection

Custom markers (@pytest.mark.slow) and selective test execution. Details: references/markers.md

Async Testing

Testing async functions with pytest-asyncio. Details: references/async_testing.md

Test Organization

Directory structure, conftest.py, and grouping related tests. Details: references/test_organization.md

Quick Reference

Running Tests

# Run all tests
pytest

# Run with coverage
pytest --cov=mypackage --cov-report=html

# Run only fast tests
pytest -m "not slow"

# Run with verbose output
pytest -v

# Run until first failure
pytest -x

# Run last failed tests
pytest --lf

Best Practices

DO:

• Follow TDD (write tests before code)
• Test one thing per test
• Use descriptive test names
• Use fixtures to eliminate duplication
• Mock external dependencies
• Aim for 80%+ coverage
• Keep tests fast

DON'T:

• Test implementation details
• Use complex conditionals in tests
• Share state between tests
• Catch exceptions in tests (use pytest.raises)
• Write tests that are too brittle

Details: references/best_practices.md

Common Patterns

API Endpoint Testing

@pytest.fixture
def client():
    app = create_app(testing=True)
    return app.test_client()

def test_get_user(client):
    response = client.get("/api/users/1")
    assert response.status_code == 200

Database Testing

@pytest.fixture
def db_session():
    session = Session(bind=engine)
    session.begin_nested()
    yield session
    session.rollback()
    session.close()

More patterns: references/common_patterns.md