Multiple event loops in custom handlers

[jordaneremieff]

Discussed in #256

^{Originally posted by jrobbins-LiveData March 20, 2022}
The Mangum doc shows this example of how one might handle a custom event:

def handler(event, context):
    if event.get("some-key"):
        # Do something or return, etc.
        return

    asgi_handler = Mangum(app)
    response = asgi_handler(event, context) # Call the instance with the event arguments

    return response

I need to handle an incoming AWS EventBridge event. I want to invoke the same method that my HTTP API handler is also invoking -- and it is an async method.

Here's what I am using

def handler(event: LambdaEvent, context: LambdaContext) -> dict:
    if "requestContext" in event:
        logger.debug("HTTP API")
        response = fastapi_handler(event, context)
    else:
        logger.debug("Not HTTP API")
        loop = asyncio.get_event_loop()
        response = loop.run_until_complete(app_logic())
    return response

My question(s): Is this the correct pattern to use? In the Mangum source, I see code like this, so I am trying to fit in. My code seems to work, but I read in the Python docs that get_event_loop() is

Deprecated since version 3.10: Deprecation warning is emitted if there is no running event loop. In future Python releases, this function will be an alias of get_running_loop().

Which made me wonder if I were doing this correctly?

[jrobbins-LiveData]

To be clear, in the simplest case of a Lambda function with a Mangum-based app (e.g. using FastAPI), there only needs to be one event loop, not multiple event loops. The only issue is making it explicit as to whom is responsible for creating the asyncio event loop. The discussion has details of how we might address this.