starlette/docs/events.md

Starlette applications can register multiple event handlers for dealing with code that 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 can be 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('shutdown')
async def close_database_connection_pool():
    ...

Or as a regular function call:

from starlette.applications import Starlette


app = Starlette()

async def open_database_connection_pool():
    ...

async def close_database_connection_pool():
    ...

app.add_event_handler('startup', open_database_connection_pool)
app.add_event_handler('shutdown', close_database_connection_pool)

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

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

Running event handlers in tests

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

Alternatively, you can use TestClient as a context manager, to ensure that startup and shutdown events are called.

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


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

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