pytest

# Pytest Pytest is the dominant testing framework for Python. It is loved for its no-boilerplate syntax (`assert x == y`), powerful fixture system, and rich plugin ecosystem. ## When to Use - **Python Standard Testing Framework**: The most widely adopted, powerful testing framework for Python web backends, data science, and ML. - **Clean Fixture Dependency Injection**: Managing test setup, database connections, and teardown cleanly with `@pytest.fixture`. - **Parameterized Testing**: Running test functions across multi-dimensional input sets with `@pytest.mark.parametrize`. - **Rich Plugin Ecosystem**: Extending capabilities via `pytest-asyncio`, `pytest-cov`, `pytest-xdist`, and `pytest-mock`. ## Quick Start ```python # content of test_sample.py def inc(x): return x + 1 def test_answer(): assert inc(3) == 5 ``` Running it: ```bash pytest ``` ## Core Concepts ### Dependency Injection with Fixtures Fixtures provide modular, reusable dependencies with explicit lifecycle scopes: ```python # conftest.py import pytest from sqlalchemy import create_engine from sqlalchemy.orm import sessionmaker @pytest.fixture(scope="session") def db_engine(): engine = create_engine("sqlite:///:memory:") yield engine engine.dispose() @pytest.fixture def db_session(db_engine): Session = sessionmaker(bind=db_engine) session = Session() yield session session.rollback() session.close() ``` ### Parameterized Tests (`@pytest.mark.parametrize`) Tests multiple input/output scenarios cleanly without loop boilerplate: ```python import pytest @pytest.mark.parametrize("email,expected_valid", [ ("user@example.com", True), ("invalid-email", False), ("@missinguser.com", False), ("user.name+tag@sub.domain.co", True), ]) def test_email_validation(email, expected_valid): assert validate_email(email) == expected_valid ``` ### Async Testing with `pytest-asyncio` Tests async coroutines seamlessly: ```python import pytest import httpx @pytest.mark.asyncio async def test_async_fetch(): async with httpx.AsyncClient() as client: response = await client.get("https://httpbin.org/get") assert response.status_code == 200 ``` ## Common Patterns ### Scoped Fixtures with Teardown / Cleanup **Problem**: Database or mock setup code repeated across dozens of test functions creates duplication and leftover state. **Solution**: Use `@pytest.fixture` with the `yield` statement: ```python import pytest @pytest.fixture(scope="function") def db_session(): # Setup test database session session = create_test_session() yield session # Cleanup and rollback after test completes session.rollback() session.close() def test_user_creation(db_session): user = db_session.create_user("alice@example.com") assert user.id is not None ``` ## Best Practices **Do**: - Use Plain `assert` Statements: Pytest provides detailed assertion introspection without requiring special assertion methods. - Use `conftest.py` for Shared Fixtures: Place global fixtures and configuration hooks in `conftest.py` files. - Run Parallel Tests with `pytest-xdist`: Accelerate test runs across all CPU cores with `pytest -n auto`. - Mark Tests by Category: Use custom marks (`@pytest.mark.slow`, `@pytest.mark.integration`) and filter runs with `-m "not slow"`. **Don't**: - Use `assert` with parentheses: Writing `assert(a == b, "msg")` evaluates a non-empty tuple which is always truthy. - Mutate shared session-scoped fixtures: Keep session fixtures read-only; use function-scoped fixtures for test-isolated mutations. - Create deep nested class hierarchies: Pytest favors simple standalone test functions over unittest-style classes. ## Troubleshooting | Error | Cause | Solution | | :------------------------------------------- | :---------------------------------------------------------------------- | :--------------------------------------------------------------------------------------------------- | | `ModuleNotFoundError: No module named 'src'` | Current directory not in `sys.path` when running pytest. | Run pytest as a module: `python -m pytest`, or configure `pythonpath = ["src"]` in `pyproject.toml`. | | `FixtureNotFound: fixture '...' not found` | Typo in fixture argument name or fixture defined outside `conftest.py`. | Move shared fixtures to root `conftest.py` file. | | `Failed: DID NOT RAISE <class '...'>` | Code did not raise the exception expected by `pytest.raises`. | Inspect inputs to verify failure condition is actually triggered. | ## References - [Pytest Documentation](https://docs.pytest.org/)