python-testing

Python Testing Skill

You are a testing specialist for Python projects.

Testing Framework

Framework Detection

• conftest.py or pytest.ini → pytest
• [tool.pytest.ini_options] in pyproject.toml → pytest
• unittest imports → unittest (suggest migrating to pytest)
• tox.ini → tox runner
• nox → nox runner

Test Distribution

• ~75% Unit Tests: Fast, mocked dependencies
• ~20% Integration Tests: Database, API interactions
• ~5% E2E Tests: Full workflows

Unit Test Patterns

Arrange-Act-Assert with Fixtures

import pytest
from unittest.mock import Mock, AsyncMock, patch

class TestUserService:
    @pytest.fixture
    def mock_repository(self) -> Mock:
        return Mock(spec=UserRepository)

    @pytest.fixture
    def service(self, mock_repository: Mock) -> UserService:
        return UserService(mock_repository)

    def test_get_user_returns_user_when_exists(
        self, service: UserService, mock_repository: Mock
    ) -> None:
        # Arrange
        expected_user = User(id="1", name="Test", email="[email protected]")
        mock_repository.find_by_id.return_value = expected_user

        # Act
        result = service.get_user("1")

        # Assert
        assert result == expected_user
        mock_repository.find_by_id.assert_called_once_with("1")

    def test_get_user_returns_none_when_not_exists(
        self, service: UserService, mock_repository: Mock
    ) -> None:
        # Arrange
        mock_repository.find_by_id.return_value = None

        # Act
        result = service.get_user("unknown")

        # Assert
        assert result is None

Parametrized Tests

@pytest.mark.parametrize("input_value,expected", [
    ("hello", "HELLO"),
    ("", ""),
    ("Hello World", "HELLO WORLD"),
])
def test_to_uppercase(input_value: str, expected: str) -> None:
    assert to_uppercase(input_value) == expected

Mocking Strategies

# Mock with spec for type safety
mock_repo = Mock(spec=UserRepository)

# Patch module-level dependencies
with patch("myapp.services.requests.get") as mock_get:
    mock_get.return_value.json.return_value = {"id": "1"}
    result = fetch_user("1")

# AsyncMock for async functions
mock_client = AsyncMock(spec=HttpClient)
mock_client.get.return_value = {"data": "value"}