Allow user provided handlers

[cblegare]

Hello there!

While investigating #176 I found it could be useful to add user provided handler factories to Mangum. It could look like

    def __init__(
        self, 
        app: ASGIApp, 
        lifespan: str = "auto", 
        additional_handler_factories: Optional[List[Callable[[event: dict, context: "LambdaContext", Dict[str, Any]], AbstractHandler]]] = None
        **handler_kwargs: Dict[str, Any]
    ) -> None:
        self.app = app
        self.lifespan = lifespan
        self.additional_handler_factories = additional_handle_factories or []
        self.additional_handler_factories.append(AbstractHandler.from_trigger)
        self.handler_kwargs = handler_kwargs

        if self.lifespan not in ("auto", "on", "off"):
            raise ConfigurationError(
                "Invalid argument supplied for `lifespan`. Choices are: auto|on|off"
            )


    def __call__(self, event: dict, context: "LambdaContext") -> dict:
        logger.debug("Event received.")


        with ExitStack() as stack:
            if self.lifespan != "off":
                lifespan_cycle: ContextManager = LifespanCycle(self.app, self.lifespan)
                stack.enter_context(lifespan_cycle)

            for handler_factory in self.additional_handler_factories:
                handler = handler_factory(event, context, **self.handler_kwargs)
                if handler:
                    break
            else:
                raise TypeError("Unable to determine handler from trigger event")

            http_cycle = HTTPCycle(handler.request)
            response = http_cycle(self.app, handler.body)


        return handler.transform_response(response)

In place of:

https://github.com/jordaneremieff/mangum/blob/3f312acb67aac30c07dfb305d3cd1881c59755c4/mangum/adapter.py#L47-L73

In order to accept user-provider handler factories. Would you be interested in this?

[jordaneremieff]

@cblegare I like this idea, thanks for proposing it.

I started working on a handler for WebSockets that might impact this design, so I think after I have a chance to get the initial WebSocket handler implemented that we can see how to work in configurable handlers - there's a bit of refactoring needed first.

It might be a week or two before I can look into this further.

[jordaneremieff]

I've introduced this provisionally in a recent PR using a method similar to what you've demonstrated. It's provisional/undocumented, but you can find an example in https://github.com/jordaneremieff/mangum/blob/main/tests/handlers/test_custom.py. I'm not sure how this will ultimately end up, so I'll leave this issue open for discussion/until I'm sure whether or not to support this going forward.

[jordaneremieff]

This feature is now added. Not considered provisional at this point.