Asynchronous sleep statements gets cancelled instantly when used with uvicorn and starlette

[Mys7erio]

In my project I have a function which must use asyncio.sleep() to perform an async no op. I have the following code to mimic the issue:

Working Code

from starlette.applications import Starlette
from starlette.responses import StreamingResponse
from starlette.routing import Route
import uvicorn
import random
import asyncio

async def homepage(request):
    video = open("C:\\Users\\USERNAME\\Downloads\\Big Buck Bunny\\Big Buck Bunny.mp4", 'rb')

    async def stream_generator():
        while True:
            try:
                if random.random() * 100 > 99:
                    print("Working fineeeeeeeeeeeeeeeeeeeeeeeeeeeeeee")
            except asyncio.CancelledError:
                print("Caught the Culpritttttttttttttttttttttttttttt")

            yield video.read(16384)

    return StreamingResponse(stream_generator(), media_type='video/mp4')

app = Starlette(debug=True, routes=[
    Route('/', homepage),
])


if __name__ == "__main__":
    uvicorn.run(app, host="0.0.0.0", port=80, log_level="info")

Output

PS D:\work> python .\utils\test.py
INFO:     Started server process [5184]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://0.0.0.0:80 (Press CTRL+C to quit)
INFO:     127.0.0.1:51189 - "GET / HTTP/1.1" 200 OK
Working fineeeeeeeeeeeeeeeeeeeeeeeeeeeeeee
Working fineeeeeeeeeeeeeeeeeeeeeeeeeeeeeee
INFO:     127.0.0.1:51190 - "GET / HTTP/1.1" 200 OK
Working fineeeeeeeeeeeeeeeeeeeeeeeeeeeeeee
Working fineeeeeeeeeeeeeeeeeeeeeeeeeeeeeee
Working fineeeeeeeeeeeeeeeeeeeeeeeeeeeeeee
Working fineeeeeeeeeeeeeeeeeeeeeeeeeeeeeee

---

The above given code works fine and successfully streams the video file off of disk. The problem arises when I put in asyncio.sleep() statements in the stream generator function:

This code works but cancels the sleep statements

from starlette.applications import Starlette
from starlette.responses import JSONResponse, StreamingResponse
from starlette.routing import Route
import uvicorn
import random
import asyncio

async def homepage(request):
    video = open("C:\\Users\\USERNAME\\Downloads\\Big Buck Bunny\\Big Buck Bunny.mp4", 'rb')

    async def stream_generator():
        while True:
            try:
                if random.random() * 100 > 99:
                    await asyncio.sleep(1)
                    print("Working fineeeeeeeeeeeeeeeeeeeeeeeeeeeeeee")
            except asyncio.CancelledError:
                print("Caught the Culpritttttttttttttttttttttttttttt")

            yield video.read(16384)

    return StreamingResponse(stream_generator(), media_type='video/mp4')


app = Starlette(debug=True, routes=[
    Route('/', homepage),
])


if __name__ == "__main__":
    uvicorn.run(app, host="0.0.0.0", port=80, log_level="info")

Output

PS D:\work> python .\utils\test.py
INFO:     Started server process [5184]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://0.0.0.0:80 (Press CTRL+C to quit)
INFO:     127.0.0.1:51189 - "GET / HTTP/1.1" 200 OK
Caught the Culpritttttttttttttttttttttttttttt
Caught the Culpritttttttttttttttttttttttttttt
Caught the Culpritttttttttttttttttttttttttttt
                [Output Shortened]
Caught the Culpritttttttttttttttttttttttttttt
Caught the Culpritttttttttttttttttttttttttttt

Also the stream generator function works fine when I simply print out the bytes yielded by stream_generator() function so I'm suspecting it's either uvicorn or starlette which is causing this unexpected behaviour.

StackOverflow link for this question

[Kludex]

What's the version of uvicorn and starlette? If it's not the latest, can you try this again on the latest?

[Mys7erio]

I am running uvicorn v0.20.0 and starlette v0.22.0 which are the latest versions available via pip

[Mys7erio]

Also can you confirm being able to reproduce the issue with the above given code?

[Kludex]

I need an MRE.

[Mys7erio]

Here's the MRE. This needs an actual video file to work:

from starlette.applications import Starlette
from starlette.responses import JSONResponse, StreamingResponse
from starlette.routing import Route
import uvicorn
import random
import asyncio

async def stream_generator():
    # Change the following file location to an actual video file on your disk
    video = open("C:\\Users\\Shakir\\Downloads\\Big Buck Bunny\\Big Buck Bunny.mp4", 'rb')
    while True:
        try:
            # This statement always gets cancelled
            await asyncio.sleep(1)
        except asyncio.CancelledError:
            # The asyncio.CancelledError exception gets caught here
            print("Caught the Culpritttttttttttttttttttttttttttt")
        yield video.read(16384)


async def homepage(request):
    return StreamingResponse(stream_generator(), media_type='video/mp4')


app = Starlette(debug=True, routes=[
    Route('/', homepage),
])


if __name__ == "__main__":
    uvicorn.run(app, host="0.0.0.0", port=80, log_level="info")

[Mys7erio]

Also, I've tested and it turns out that the asyncio.sleep statements get cancelled only when I'm returning a StreamingResponse. I've tried using the same sleep statement before returning a simple JSONResponse and it worked.

[euri10]

you cant use asyncio.sleep() , use asyncio.sleep(0)

[Mys7erio]

Sleeping for 0 seconds defeats the whole point of sleeping, so I can't accept this answer. Also, in my examples, I have slept for atleast for 1 or 3 seconds.

[euri10]

you say you want to perform an async no op that is what asyncio.sleep(0) is for

[Mys7erio]

Sorry for that, I worded it wrong. I want the program to execute an asynchronous sleep statement but it gets cancelled (probably) because of uvicorn. I need some help finding why this is happening.

Besides that, I'll also appreciate if you know any other way of doing an async no op without using asyncio.sleep()

[Mys7erio]

I upgraded to Python 3.11 and the issue seems to be fixed. Also it may be that having a generator inside an async function is a bad idea, and that't what was causing all the problems.

This issue can be closed now.