Startup and Shutdown

The ASGI lifespan specification includes the ability for awaiting coroutines before the first byte is received and after the final byte is sent, through the startup and shutdown lifespan events. This is particularly useful for creating and destroying connection pools. Quart supports this via the decorators before_serving(), after_serving(), and while_serving() which expects a function that returns a generator.

The decorated functions are all called within the app context, allowing current_app and g to be used.

Warning

Use g with caution, as it will reset after startup, i.e. after all the before_serving functions complete and after the initial yield in a while serving generator. It can still be used within this context. If you want to create something used in routes, try storing it on the app instead.

To use this functionality simply do the following:

@app.before_serving
async def create_db_pool():
    app.db_pool = await ...
    g.something = something

@app.before_serving
async def use_g():
    g.something.do_something()

@app.while_serving
async def lifespan():
    ...  # startup
    yield
    ...  # shutdown

@app.route("/")
async def index():
    app.db_pool.execute(...)
    # g.something is not available here

@app.after_serving
async def create_db_pool():
    await app.db_pool.close()

Testing

Quart’s test client works on a request lifespan and hence does not call before_serving, or after_serving functions, nor advance the while_serving generator. Instead Quart’s test app can be used, for example

@pytest.fixture(name="app", scope="function")
async def _app():
    app = create_app()  # Initialize app
    async with app.test_app() as test_app:
        yield test_app

The app fixture can then be used as normal, knowing that the before_serving, and after_serving functions have been called, and the while_serving generator has been advanced,

async def test_index(app):
    test_client = app.test_client()
    await test_client.get("/")
    ...