Try/catch is failing to catch an exception when directly inside route handler

[pamelafox]

I've put together a demonstration repository here:
https://github.com/pamelafox/quart-exception-example

The important code is:

async def chat_coroutine():
    for i in range(5):
        yield {"choices": [{"delta": {"content": str(i/0)}}]}
        await asyncio.sleep(1)

@bp.post("/chat")
async def chat_handler():

    async def response_stream():
        async for event in chat_coroutine():
            yield json.dumps(event, ensure_ascii=False) + "\n"

    try:
        return Response(response_stream())
    except Exception as e:
        return jsonify({"choices": [{"delta": {"content": str(e)}}]})

When I run that, I expect the exception (division by zero) to get caught, and for it to return json with the error message.

I instead see a 500 with this traceback:

ERROR:hypercorn.error:Error in ASGI Framework
Traceback (most recent call last):
  File "/home/vscode/.local/lib/python3.11/site-packages/hypercorn/asyncio/task_group.py", line 23, in _handle
    await app(scope, receive, send, sync_spawn, call_soon)
  File "/home/vscode/.local/lib/python3.11/site-packages/hypercorn/app_wrappers.py", line 33, in __call__
    await self.app(scope, receive, send)
  File "/home/vscode/.local/lib/python3.11/site-packages/quart/app.py", line 1902, in __call__
    await self.asgi_app(scope, receive, send)
  File "/home/vscode/.local/lib/python3.11/site-packages/quart/app.py", line 1928, in asgi_app
    await asgi_handler(receive, send)
  File "/home/vscode/.local/lib/python3.11/site-packages/quart/asgi.py", line 51, in __call__
    _raise_exceptions(done)
  File "/home/vscode/.local/lib/python3.11/site-packages/quart/asgi.py", line 353, in _raise_exceptions
    raise task.exception()
  File "/home/vscode/.local/lib/python3.11/site-packages/quart/asgi.py", line 102, in handle_request
    await asyncio.wait_for(self._send_response(send, response), timeout=timeout)
  File "/usr/local/lib/python3.11/asyncio/tasks.py", line 489, in wait_for
    return fut.result()
           ^^^^^^^^^^^^
  File "/home/vscode/.local/lib/python3.11/site-packages/quart/asgi.py", line 131, in _send_response
    async for data in response_body:
  File "/workspaces/quart-exception-example/src/quartapp/chat.py", line 23, in response_stream
    async for event in chat_coroutine():
  File "/workspaces/quart-exception-example/src/quartapp/chat.py", line 16, in chat_coroutine
    yield {"choices": [{"delta": {"content": str(i/0)}}]}
                                                 ~^~
ZeroDivisionError: division by zero

I'm trying to understand if this is to be expected, due to some consequence of async generators, or if this is an issue in Quart.

I do get the exception handled if I move the try/catch inside response_stream, but was hoping to use it a level up (as this is a simplification of a more complex codebase). Thanks!

[pgjones]

I'm not sure this is possible in general, as return Response(response_stream()) creates and defines a 200 streamed response i.e. once created it can't be changed within the generator to a json error response.

[pamelafox]

Yeah, good point! I'm curious how other APIs/apps convey errors that happen in the midst of a streamed response. 200 basically means "OK the stream started but errors might happen later" so the frontend needs to prepare for that. Very interesting. Thanks for replying :)