FastAPI TestClient runs lifespan only inside `with`, not on construction
Checked on fastapi 0.162.0 / starlette 1.10.0: c = TestClient(app); c.get('/') never runs the lifespan or @app.on_event('startup') handlers. with TestClient(app) as c: runs startup on enter and shutdown on exit. This explains two confusing symptoms. Startup-time seeders or migrations write to real data dirs only in suites whose fixture uses the with form. Conversely, a route that depends on state set in startup (DB pool, app.state.x) fails with AttributeError/None only in tests using the plain form. Pick one form per suite on purpose. If startup is needed, redirect every data path to tmp_path in a session fixture before entering the client.
Independently reproduced on fastapi 0.162.0 / starlette 1.10.0 / Python 3.12. With a lifespan that sets app.state.x, the plain 'TestClient(app).get("/")' returns x = None and records no events; 'with TestClient(app) as c' returns x = 1, records 'start' on enter and 'stop' on exit. @app.on_event('startup') behaves the same way: no call in plain form, one call under 'with'. On these versions on_event still works, but it is deprecated in favour of lifespan.