---

name: python-testing description: Write and run Python tests using pytest with fixtures, mocking, coverage, and the AAA pattern. Use when writing Python tests, creating test files, or setting up pytest configuration.

Python Testing Skill

When to Activate

Activate this skill when:

• Writing Python unit tests
• Creating test files or test directories
• Setting up pytest configuration
• Working with test fixtures or mocking
• Running tests or checking coverage

Quick Commands

Copy# Run all tests
uv run pytest

# Run specific file
uv run pytest tests/test_auth.py

# Run with coverage
uv run pytest --cov=src --cov-report=html

# Run verbose with output
uv run pytest -v -s

# Run tests matching pattern
uv run pytest -k "test_user"

# Run last failed
uv run pytest --lf

Test Structure: AAA Pattern

Follow Arrange-Act-Assert for clear, maintainable tests:

Copydef test_user_registration():
    # Arrange - set up test data
    email = "test@example.com"
    password = "secure_pass123"
    user_service = UserService()

    # Act - execute the function
    result = user_service.register(email, password)

    # Assert - verify the outcome
    assert result.success is True
    assert result.user.email == email

Directory Structure

Copyproject/
├── src/
│   └── myapp/
├── tests/
│   ├── conftest.py      # Shared fixtures
│   ├── unit/
│   │   └── test_utils.py
│   ├── integration/
│   │   └── test_api.py
│   └── e2e/
│       └── test_workflows.py
└── pyproject.toml

Fixtures (conftest.py)

Copyimport pytest
from myapp import create_app

@pytest.fixture
def app():
    """Create application for testing."""
    app = create_app("testing")
    yield app

@pytest.fixture
def client(app):
    """Create test client."""
    return app.test_client()

@pytest.fixture
def sample_user():
    """Create a sample user for testing."""
    return User(email="test@example.com", username="testuser")

Parametrized Tests

Copy@pytest.mark.parametrize("email,expected", [
    ("valid@example.com", True),
    ("invalid.email", False),
    ("special+chars@test.co.uk", True),
])
def test_email_validation(email, expected):
    assert is_valid_email(email) == expected

Mocking

Copyfrom unittest.mock import Mock, patch

def test_api_call():
    mock_response = Mock()
    mock_response.json.return_value = {"data": "success"}

    with patch("requests.get", return_value=mock_response):
        result = fetch_data("https://api.example.com")
        assert result == {"data": "success"}

Common Patterns

Testing Exceptions

Copydef test_division_by_zero():
    with pytest.raises(ValueError, match="Cannot divide by zero"):
        divide(10, 0)

Testing with Temp Files

Copydef test_file_processing(tmp_path):
    test_file = tmp_path / "test.txt"
    test_file.write_text("Hello World")
    result = process_file(str(test_file))
    assert result.word_count == 2

Testing Environment Variables

Copydef test_config_from_env(monkeypatch):
    monkeypatch.setenv("API_KEY", "test_key_123")
    assert load_config().api_key == "test_key_123"

Coverage Configuration

Copy# pyproject.toml
[tool.pytest.ini_options]
testpaths = ["tests"]
addopts = "-v --cov=src --cov-report=term-missing"

[tool.coverage.run]
source = ["src"]
omit = ["tests/*"]

Naming Conventions

• Test files: test_*.py
• Test functions: test_*
• Test classes: Test*

Related Resources

See AgentUsage/testing_python.md for complete documentation including:

• Async testing patterns
• Integration test examples
• CI/CD integration
• Test priority guidelines