Break from Python async for

Question

I have a situation where I want to break from an async for loop. I managed to reduce the issue to the application below. I expect to enter the 'finally' section of the context manager when breaking from the loop in main. In other words, the expected result is

try

456

finally

done

but what I get is

try

456

done

finally

and then an exception when the application closes.

Here is the code

import asyncio
from contextlib import asynccontextmanager

@asynccontextmanager
async def receiving():
    try:
        print('try')
        yield 123
    finally:
        print('finally')

async def request_all():
    async with receiving():
        yield 456

async def main():
    async for r in request_all():
        print(r)
        break
    print('done')

asyncio.run(main())

I found this bug report that seems similar, but as far as I can tell it has been resolved before 3.8. I tested my issue on 3.8.2 and 3.9.6

Answer 1

I can see how this behaviour is confusing, but I don't think it's a bug. If you bypass the exhaustion of the async Iterator with break, you never leave the async with in request_all, so the finally-block will not get executed until the event loop finishes. This has the advantage that you could exhaust the generator at a later point.

If you are sure, you don't need your generator anymore, you could close the async_generator instead of breaking to get your expected behaviour:

import asyncio
from contextlib import asynccontextmanager

@asynccontextmanager
async def receiving():
    try:
        print('try')
        yield 123
    finally:
        print('finally')

async def request_all():
    async with receiving():
        yield 456

async def main():
    gen = request_all()
    async for r in gen:
        print(r)
        await gen.aclose() #instead of break
    print('done')

asyncio.run(main())