python-testing-strategies

Python Testing Strategies — Quick Reference

Setup (CRITICAL)

Set asyncio_mode = "auto" in pyproject.toml to auto-detect async tests without markers. (The default "strict" mode also works if you mark tests with @pytest.mark.asyncio.)

See examples.md for the complete conftest.py and pyproject.toml setup.

Test Client

Use httpx.AsyncClient + ASGITransport — never TestClient for async tests.

See examples.md for client fixture setup.

Required Coverage Per Endpoint

1. Happy path (create, read, update, delete)
2. Not found (404)
3. Validation error (422)
4. Auth failure (401) — if protected
5. Soft delete — deleted records not returned

Test Naming

async def test_create_item():           # Happy path
async def test_create_item_missing_name():  # Validation
async def test_get_item_not_found():    # 404
async def test_get_item_soft_deleted(): # Soft delete

Test Data Factories

Use simple factory functions with sensible defaults. Override only what the test cares about.

See examples.md for factory patterns.

Running Tests

pytest                           # All tests
pytest tests/test_items.py       # One file
pytest -k "test_create"          # By name pattern
pytest --tb=short -q             # Quiet output

Gotchas

• Missing `asyncio_mode = "auto"` silently breaks everything. Tests appear to pass (0 collected) or hang indefinitely. Add [tool.pytest.ini_options] asyncio_mode = "auto" to pyproject.toml before writing any async test. This is the #1 cause of "my tests don't run."

• `TestClient` and `AsyncClient` are not interchangeable. TestClient (from Starlette) is synchronous — it blocks. AsyncClient (from httpx) is async — it uses the event loop. If your app uses async def routes with await, you must use AsyncClient with ASGITransport. Using TestClient may hide async bugs because it runs synchronously.

• Database state leaks between tests without proper cleanup. Each test needs a clean database. Use an autouse=True fixture that creates/drops tables, or truncate between tests. Without this, test order matters and random failures appear in CI.

• `response.json()` returns snake_case from FastAPI by default. If your Pydantic models use alias_generator, the JSON keys may differ from your Python field names. Always assert against the actual JSON keys, not the Python attribute names.

• Forgetting to `await` in async tests gives confusing errors. If you see <coroutine object ...> in test output instead of actual data, you forgot an await. Every client.get(), client.post(), etc. must be awaited.