Sending Data From An Async Generator To A Websocket

[iwoloschin]

Description
I'm attempting to stream a text log file through a websocket (think tail -f but through a browser). I have a working async generator that appears to read my file's contents in near realtime, but it appears that the await websocket.send_json(data) doesn't do anything until the async generator ends, and then everything that has been queued up gets dumped to the websocket. Is this expected behavior? If it is expected, are there any ways to work around this?

async for data in read_log(log_filename):
    log.critical(data)
    await websocket.send_json({"data": data})

In my FastAPI log I see data printed in realtime, but nothing appears on the other end of the websocket until read_log returns (instead of yields). After read_log returns, all of the log file's contents are dumped to the websocket and appear on the other side as expected. I was hoping to get data to show up as it's printed into the log file, same as you'd expect with tail -f.

Additional context
The intended use case here is to simulate a tail -f in an Angular SPA, but it can be tested by using the websocat tool (https://github.com/vi/websocat).

Also, this is my first foray into async python, so I could just be doing everything completely wrong! I'm pretty sure I'm not...but I really have no idea what I'm doing here :).

[euri10]

maybe dumb question but are you sure read_log(log_filename) is async ?
from memory aiofiles is a nice library to use for that.

[iwoloschin]

Nope, not a stupid question, looks like that was the problem. I was thrown off since I was seeing the right data in my console log so I assumed it was doing the right thing. Looks like aiofiles does the right thing though, thanks!

[tiangolo]

Great to see you solved your problem @iwoloschin.

You rock @euri10! Thanks for all your help here.

[CurryGaifan]

How to solve this problem please. @iwoloschin

[CurryGaifan]

I solved this problem. I used time.sleep incorrectly in async method.
The code below can work.

async def get_text():
    for i in range(5):
        yield i
        # time.sleep(2)  
        await asyncio.sleep(5)

async for text in get_text():
    await websocket.send_text(f"Message text was: {text}")
    logger.info(text)

[JonathanChavezTamales]

Just for further reference, I had the same problem but I wanted to stream the generator as soon as it comes, without a delay.

Like @CurryGaifan, I had to add await asyncio.sleep(0), but with 0 seconds. My guess is that this allows the websocket buffer to flush.