Python Testing Patterns

Robust testing with pytest.

Before Writing Tests

1. Read references/pythonic-style.md for naming and style conventions
2. TDD workflow: Write failing test first, then implement
3. Test behavior, not implementation details

Reference Files

Basic Tests

import pytest

def test_addition():
    assert 2 + 2 == 4

def test_exception_raised():
    with pytest.raises(ValueError, match="invalid"):
        raise ValueError("invalid input")

Fixtures

@pytest.fixture
def db():
    """
    Provides isolated database connection for test isolation.

    Using in-memory SQLite prevents test pollution across runs and
    guarantees each test starts with known state.
    """
    database = Database("sqlite:///:memory:")
    database.connect()
    yield database
    database.disconnect()

def test_query(db):
    results = db.query("SELECT * FROM users")
    assert len(results) >= 0

# Session scope shares fixture across all tests for expensive resources
# (e.g., one DB schema creation for all tests vs per-test).
@pytest.fixture(scope="session")
def app_config():
    return {"debug": True, "db_url": "sqlite:///:memory:"}

Parametrized Tests

@pytest.mark.parametrize("email,valid", [
    ("user@example.com", True),
    ("invalid.email", False),
    ("", False),
])
def test_email_validation(email, valid):
    assert is_valid_email(email) == valid

# With custom IDs
@pytest.mark.parametrize("value,expected", [
    pytest.param(1, True, id="positive"),
    pytest.param(0, False, id="zero"),
    pytest.param(-1, False, id="negative"),
])
def test_is_positive(value, expected):
    assert (value > 0) == expected

Mocking

from unittest.mock import Mock, patch

def test_api_call():
    mock_response = Mock()
    mock_response.json.return_value = {"id": 1, "name": "Test"}

    with patch("requests.get", return_value=mock_response) as mock_get:
        result = fetch_user(1)

        assert result["id"] == 1
        mock_get.assert_called_once_with("https://api.example.com/users/1")

# Decorator syntax
@patch("module.external_service")
def test_with_mock(mock_service):
    mock_service.return_value = "mocked"
    assert my_function() == "mocked"

Async Testing

import pytest

@pytest.mark.asyncio
async def test_async_fetch():
    result = await fetch_data("https://api.example.com")
    assert result is not None

@pytest.mark.asyncio
async def test_timeout():
    with pytest.raises(asyncio.TimeoutError):
        await asyncio.wait_for(slow_operation(), timeout=0.1)

# Async fixture
@pytest.fixture
async def async_client():
    client = AsyncClient()
    await client.connect()
    yield client
    await client.disconnect()

Test Organization

tests/
├── conftest.py           # Shared fixtures
├── test_unit/
│   ├── test_models.py
│   └── test_utils.py
├── test_integration/
│   └── test_api.py
└── test_e2e/
    └── test_workflows.py

Naming Conventions

# Good: describes behavior and expected outcome
def test_user_creation_with_valid_data_succeeds():
def test_login_fails_with_invalid_password():
def test_api_returns_404_for_missing_resource():

# Bad: vague or non-descriptive
def test_user():
def test_1():

Markers

@pytest.mark.slow
def test_slow_operation():
    ...

@pytest.mark.integration
def test_database():
    ...

@pytest.mark.skip(reason="Not implemented")
def test_future_feature():
    ...

@pytest.mark.xfail(reason="Known bug #123")
def test_known_bug():
    ...

# Run: pytest -m "not slow"

Configuration

# pyproject.toml
[tool.pytest.ini_options]
testpaths = ["tests"]
addopts = ["-v", "--cov=myapp", "--cov-report=term-missing"]
markers = [
    "slow: marks tests as slow",
    "integration: marks integration tests",
]

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

TDD Workflow

1. RED: Write failing test
2. GREEN: Write minimal code to pass
3. REFACTOR: Improve while keeping tests green

# 1. Write test first
def test_calculate_total():
    cart = Cart([Item(10), Item(20)])
    assert cart.total() == 30

# 2. Run test - it fails (Cart doesn't exist)

# 3. Implement minimal code
class Cart:
    def __init__(self, items):
        self.items = items
    def total(self):
        return sum(item.price for item in self.items)

# 4. Run test - it passes

# 5. Refactor if needed (tests stay green)

Workflow Integration

If the core plugin is installed, use these skills:

Best Practices

1. One assertion per test when possible
2. Descriptive names that explain behavior
3. Independent tests - no shared mutable state
4. Use fixtures for setup/teardown
5. Mock external dependencies appropriately
6. Parametrize to reduce duplication
7. Test edge cases and error conditions
8. Run tests in CI on every commit
9. Measure coverage but focus on quality
10. Write tests first (TDD) when possible