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

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:

def 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

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

Fixtures (conftest.py)

import 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

@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

from 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

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

Testing with Temp Files

def 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

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

Coverage Configuration

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