Python Testing Patterns
Comprehensive testing strategies for Python applications using pytest, TDD methodology, and best practices.
When to Activate
- Writing new Python code (follow TDD: red, green, refactor)
- Designing test suites for Python projects
- Reviewing Python test coverage
- Setting up testing infrastructure
Core Testing Philosophy
Test-Driven Development (TDD)
Always follow the TDD cycle:
- RED: Write a failing test for the desired behavior
- GREEN: Write minimal code to make the test pass
- REFACTOR: Improve code while keeping tests green
python
# Step 1: Write failing test (RED)
def test_add_numbers():
result = add(2, 3)
assert result == 5
# Step 2: Write minimal implementation (GREEN)
def add(a, b):
return a + b
# Step 3: Refactor if needed (REFACTOR)
Coverage Requirements
- Target: 80%+ code coverage
- Critical paths: 100% coverage required
- Use
pytest --cov to measure coverage
bash
pytest --cov=mypackage --cov-report=term-missing --cov-report=html
pytest Fundamentals
Basic Test Structure
python
import pytest
def test_addition():
"""Test basic addition."""
assert 2 + 2 == 4
def test_string_uppercase():
"""Test string uppercasing."""
text = "hello"
assert text.upper() == "HELLO"
def test_list_append():
"""Test list append."""
items = [1, 2, 3]
items.append(4)
assert 4 in items
assert len(items) == 4
Assertions
python
# Equality
assert result == expected
# Inequality
assert result != unexpected
# Truthiness
assert result # Truthy
assert not result # Falsy
assert result is True # Exactly True
assert result is False # Exactly False
assert result is None # Exactly None
# Membership
assert item in collection
assert item not in collection
# Comparisons
assert result > 0
assert 0 <= result <= 100
# Type checking
assert isinstance(result, str)
# Exception testing (preferred approach)
with pytest.raises(ValueError):
raise ValueError("error message")
# Check exception message
with pytest.raises(ValueError, match="invalid input"):
raise ValueError("invalid input provided")
# Check exception attributes
with pytest.raises(ValueError) as exc_info:
raise ValueError("error message")
assert str(exc_info.value) == "error message"
Fixtures
Basic Fixture Usage
python
import pytest
@pytest.fixture
def sample_data():
"""Fixture providing sample data."""
return {"name": "Alice", "age": 30}
def test_sample_data(sample_data):
"""Test using the fixture."""
assert sample_data["name"] == "Alice"
assert sample_data["age"] == 30
Fixture with Setup/Teardown
python
@pytest.fixture
def database():
"""Fixture with setup and teardown."""
# Setup
db = Database(":memory:")
db.create_tables()
db.insert_test_data()
yield db # Provide to test
# Teardown
db.close()
def test_database_query(database):
"""Test database operations."""
result = database.query("SELECT * FROM users")
assert len(result) > 0
Fixture Scopes
python
# Function scope (default) - runs for each test
@pytest.fixture
def temp_file():
with open("temp.txt", "w") as f:
yield f
os.remove("temp.txt")
# Module scope - runs once per module
@pytest.fixture(scope="module")
def module_db():
db = Database(":memory:")
db.create_tables()
yield db
db.close()
# Session scope - runs once per test session
@pytest.fixture(scope="session")
def shared_resource():
resource = ExpensiveResource()
yield resource
resource.cleanup()
Fixture with Parameters
python
@pytest.fixture(params=[1, 2, 3])
def number(request):
"""Parameterized fixture."""
return request.param
def test_numbers(number):
"""Test runs 3 times, once for each parameter."""
assert number > 0
Using Multiple Fixtures
python
@pytest.fixture
def user():
return User(id=1, name="Alice")
@pytest.fixture
def admin():
return User(id=2, name="Admin", role="admin")
def test_user_admin_interaction(user, admin):
"""Test using multiple fixtures."""
assert admin.can_manage(user)
Autouse Fixtures
python
@pytest.fixture(autouse=True)
def reset_config():
"""Automatically runs before every test."""
Config.reset()
yield
Config.cleanup()
def test_without_fixture_call():
# reset_config runs automatically
assert Config.get_setting("debug") is False
Conftest.py for Shared Fixtures (Flask)
python
# tests/conftest.py
import pytest
@pytest.fixture
def client():
"""Shared fixture for all tests."""
app = create_app(testing=True)
with app.test_client() as client:
yield client
@pytest.fixture
def auth_headers(client):
"""Generate auth headers for API testing."""
response = client.post("/api/login", json={
"username": "test",
"password": "test"
})
token = response.json["token"]
return {"Authorization": f"Bearer {token}"}
Parametrization
Basic Parametrization
python
@pytest.mark.parametrize("input,expected", [
("hello", "HELLO"),
("world", "WORLD"),
("PyThOn", "PYTHON"),
])
def test_uppercase(input, expected):
"""Test runs 3 times with different inputs."""
assert input.upper() == expected
Multiple Parameters
python
@pytest.mark.parametrize("a,b,expected", [
(2, 3, 5),
(0, 0, 0),
(-1, 1, 0),
(100, 200, 300),
])
def test_add(a, b, expected):
"""Test addition with multiple inputs."""
assert add(a, b) == expected
Parametrize with IDs
python
@pytest.mark.parametrize("input,expected", [
("valid@email.com", True),
("invalid", False),
("@no-domain.com", False),
], ids=["valid-email", "missing-at", "missing-domain"])
def test_email_validation(input, expected):
"""Test email validation with readable test IDs."""
assert is_valid_email(input) is expected
Parametrized Fixtures
python
@pytest.fixture(params=["sqlite", "postgresql", "mysql"])
def db(request):
"""Test against multiple database backends."""
if request.param == "sqlite":
return Database(":memory:")
elif request.param == "postgresql":
return Database("postgresql://localhost/test")
elif request.param == "mysql":
return Database("mysql://localhost/test")
def test_database_operations(db):
"""Test runs 3 times, once for each database."""
result = db.query("SELECT 1")
assert result is not None
Markers and Test Selection
Custom Markers
python
# Mark slow tests
@pytest.mark.slow
def test_slow_operation():
time.sleep(5)
# Mark integration tests
@pytest.mark.integration
def test_api_integration():
response = requests.get("https://api.example.com")
assert response.status_code == 200
# Mark unit tests
@pytest.mark.unit
def test_unit_logic():
assert calculate(2, 3) == 5
Run Specific Tests
bash
# Run only fast tests
pytest -m "not slow"
# Run only integration tests
pytest -m integration
# Run integration or slow tests
pytest -m "integration or slow"
# Run tests marked as unit but not slow
pytest -m "unit and not slow"
Configure Markers in pytest.ini
ini
[pytest]
markers =
slow: marks tests as slow
integration: marks tests as integration tests
unit: marks tests as unit tests
django: marks tests as requiring Django
Mocking and Patching
Mocking Functions
python
from unittest.mock import patch, Mock
@patch("mypackage.external_api_call")
def test_with_mock(api_call_mock):
"""Test with mocked external API."""
api_call_mock.return_value = {"status": "success"}
result = my_function()
api_call_mock.assert_called_once()
assert result["status"] == "success"
Mocking Return Values
python
@patch("mypackage.Database.connect")
def test_database_connection(connect_mock):
"""Test with mocked database connection."""
connect_mock.return_value = MockConnection()
db = Database()
db.connect()
connect_mock.assert_called_once_with("localhost")
Mocking Exceptions
python
@patch("mypackage.api_call")
def test_api_error_handling(api_call_mock):
"""Test error handling with mocked exception."""
api_call_mock.side_effect = ConnectionError("Network error")
with pytest.raises(ConnectionError):
api_call()
api_call_mock.assert_called_once()
Mocking Context Managers
python
@patch("builtins.open", new_callable=mock_open)
def test_file_reading(mock_file):
"""Test file reading with mocked open."""
mock_file.return_value.read.return_value = "file content"
result = read_file("test.txt")
mock_file.assert_called_once_with("test.txt", "r")
assert result == "file content"
Using Autospec
python
@patch("mypackage.DBConnection", autospec=True)
def test_autospec(db_mock):
"""Test with autospec to catch API misuse."""
db = db_mock.return_value
db.query("SELECT * FROM users")
# This would fail if DBConnection doesn't have query method
db_mock.assert_called_once()
Mock Class Instances
python
class TestUserService:
@patch("mypackage.UserRepository")
def test_create_user(self, repo_mock):
"""Test user creation with mocked repository."""
repo_mock.return_value.save.return_value = User(id=1, name="Alice")
service = UserService(repo_mock.return_value)
user = service.create_user(name="Alice")
assert user.name == "Alice"
repo_mock.return_value.save.assert_called_once()
Mock Property
python
@pytest.fixture
def mock_config():
"""Create a mock with a property."""
config = Mock()
type(config).debug = PropertyMock(return_value=True)
type(config).api_key = PropertyMock(return_value="test-key")
return config
def test_with_mock_config(mock_config):
"""Test with mocked config properties."""
assert mock_config.debug is True
assert mock_config.api_key == "test-key"
Testing Async Code
Async Tests with pytest-asyncio
python
import pytest
@pytest.mark.asyncio
async def test_async_function():
"""Test async function."""
result = await async_add(2, 3)
assert result == 5
@pytest.mark.asyncio
async def test_async_with_fixture(async_client):
"""Test async with async fixture."""
response = await async_client.get("/api/users")
assert response.status_code == 200
Async Fixture (FastAPI)
ASGITransport does not run lifespan events. Install asgi-lifespan and wrap the client in LifespanManager when the app uses startup or shutdown handlers. See FastAPI's async testing guidance.
python
import pytest
import pytest_asyncio
from asgi_lifespan import LifespanManager
from httpx import ASGITransport, AsyncClient
@pytest_asyncio.fixture
async def async_client():
"""Async fixture providing async test client with lifespan events."""
app = create_app() # Returns a FastAPI application
async with LifespanManager(app) as manager:
async with AsyncClient(transport=ASGITransport(app=manager.app), base_url="http://test") as client:
yield client
@pytest.mark.asyncio
async def test_api_endpoint(async_client):
"""Test using async fixture."""
response = await async_client.get("/api/data")
assert response.status_code == 200
Mocking Async Functions
python
@pytest.mark.asyncio
@patch("mypackage.async_api_call")
async def test_async_mock(api_call_mock):
"""Test async function with mock."""
api_call_mock.return_value = {"status": "ok"}
result = await my_async_function()
api_call_mock.assert_awaited_once()
assert result["status"] == "ok"
Testing Exceptions
Testing Expected Exceptions