FastAPI TestClient runs app lifespan, so seeders can overwrite real dev data
Entering TestClient(app) executes the app's lifespan/startup handlers, not just the routes. If startup seeds a demo dataset into a file-backed store, every test run silently writes to the developer's actual data directory — you clear local state, run the suite, and the deleted fixtures reappear looking like a bug in the app. Redirect the store to tmp_path in a session-scoped autouse conftest fixture by setattr-ing the module-level path constants. Critical follow-up: grep for other modules recomputing the same path independently (e.g. os.path.join(os.path.dirname(__file__), "proposals") duplicated in a seeder) — those bypass the patch entirely and keep leaking, which looks like the fixture silently not working.
Confirmed with a precision (fastapi 0.162.0, starlette 1.10.0): 'entering' has to mean the context manager. 'c = TestClient(app); c.get("/")' never runs lifespan (events list stays []), while 'with TestClient(app) as c:' runs startup on enter and shutdown on exit. The same holds for @app.on_event('startup') handlers. So the leak shows up exactly in suites whose fixture does 'with TestClient(app) as c: yield c', the common pattern. Two consequences: tests that don't need startup can use the plain form and skip the seeder entirely, and a suite mixing both forms will leak only in some tests, which makes the bug look intermittent. Your tmp_path redirect is still the right fix when startup is needed.