“Exception ignored in:” while using `create_subprocess_exec` with infinite process

Question

I'm using Python 3.4.3. While my event loop is working, I'm communicating with some infinite process (tail -f in the example). Once all other tasks are done, I cancel tail task as well, wait for it to finish and close the loop.

import asyncio
import time

@asyncio.coroutine
def tailf():
    try:
        create = asyncio.create_subprocess_exec(
            'tail', '-f', '/tmp/file',
            stdout=asyncio.subprocess.PIPE,
        )
        proc = yield from create

        while True:
            l = yield from proc.stdout.readline()
            print(l)

    except BaseException as e:
        print('CAUGHT', type(e))


@asyncio.coroutine
def task():
    yield from asyncio.sleep(1)

loop = asyncio.get_event_loop()
tailf_task = loop.create_task(tailf())
loop.run_until_complete(asyncio.wait([task()]))
tailf_task.cancel()
loop.run_until_complete(asyncio.wait([tailf_task]))
loop.close()

The output is:

b'123\n'
CAUGHT <class 'concurrent.futures._base.CancelledError'>
Exception ignored in: Exception ignored in:

Do echo 123 > /tmp/file to get the same result before running the example.

It all works as expected except I got that warning at the end of the script.

I believe the reason is yield from proc.stdout.readline() being interrupted, but I would like to catch the exception in that case.

So, the question is: what am I doing wrong?

And the more broad one: how do I debug such warning next time?

If you change it to except BaseException as e:, does that catch it? — abarnert
– abarnert, Commented Apr 11, 2018 at 16:08
Thanks for the idea, fixed. The warning is still there though. — Vadim Pushtaev
– Vadim Pushtaev, Commented Apr 11, 2018 at 16:13
It was just a guess that canceling a coro written as a generator (pre-3.5 style) might raise a GeneratorExit, which is not meant to be caught with except Exception, only BaseException. But I guess that's not it. — abarnert
– abarnert, Commented Apr 11, 2018 at 16:15

Vadim Pushtaev · Accepted Answer · 2018-04-11 17:00:59Z

1

Once the coroutine that communicates with tail -f is canceled, it should terminate the process and wait for it to end (look at the except):

@asyncio.coroutine
def tailf():
    try:
        create = asyncio.create_subprocess_exec(
            'tail', '-f', '/tmp/file',
            stdout=asyncio.subprocess.PIPE,
        )
        proc = yield from create

        while True:
            l = yield from proc.stdout.readline()
            print(l)
    except asyncio.CancelledError:
        proc.terminate()
        yield from proc.wait()

You don't get the warning in Python 3.6, but you still get unterminated tail -f hanging in the process list.

answered Apr 11, 2018 at 17:00

Vadim Pushtaev

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