starlette/docs/events.md

Starlette applications can register multiple event handlers for dealing with code than needs to run before the application starts up, or when the application is shutting down.

Registering events

These event handlers can either be async coroutines, or regular syncronous functions.

The event handlers are registered with a decorator syntax, like so:

from starlette.applications import Starlette


app = Starlette()

@app.on_event('startup')
async def open_database_connection_pool():
    ...

@app.on_event('cleanup')
async def close_database_connection_pool():
    ...

Starlette will not start serving any incoming requests until all of the registered startup handlers have completed.

The cleanup handlers will run once all connections have been closed, and any in-process background tasks have completed.

Note: The ASGI lifespan protocol has only recently been added to the spec, and is only currently supported by the uvicorn server. Make sure to use the latest uvicorn release if you need startup/cleanup support.

Running event handers in tests

You might want to explicitly call into your event handlers in any test setup or test teardown code.

Alternatively, Starlette provides a context manager that ensures the lifespan events are called.

from example import app
from starlette.lifespan import LifespanContext
from starlette.testclient import TestClient


def test_homepage():
    with LifespanContext(app):
        # Application 'startup' handlers are called on entering the block.
        client = TestClient(app)
        response = client.get("/")
        assert response.status_code == 200

    # Application 'cleanup' handlers are called on exiting the block.