Detecting browser closure in GraphQL subscriptions

[imankulov]

I have a subscription function that continuously yields values. Here's a simplified example of my code:

@strawberry.type
class Subscription:
    @strawberry.subscription
    async def counter(self) -> AsyncGenerator[int, None]:
        for value in itertools.count():
            yield value
            print(value)
            await asyncio.sleep(1)

I would like to be able to detect when the browser closes the connection, and if the connection is closed, I want to print a message like "connection closed" from inside the counter function. Is there a way to achieve this with Strawberry or otherwise?

Uvicorn runs the app. Strawberry is exposed to clients via FastAPI:

fastapi_app.add_websocket_route("/graphql", GraphQL(strawberry.Schema(..., subscription=Subscription)))

I would greatly appreciate any guidance or suggestions. Thanks in advance!

[imankulov]

When the browser closes the connection, the subscription handler cancels the counter task (if I am not mistaken, here)

As outlined in Python's Task Cancellation documentation.

When a task is cancelled, asyncio.CancelledError will be raised in the task at the next opportunity. [...] In case asyncio.CancelledError is explicitly caught, it should generally be propagated when clean-up is complete.

The following modified code works for me.

@strawberry.type
class Subscription:
    @strawberry.subscription
    async def counter(self) -> AsyncGenerator[int, None]:
        try:
            for value in itertools.count():
                yield value
                print(value)
                await asyncio.sleep(1)
        except asyncio.CancelledError:
            print("Cancelled")
            raise