Python module not running in docker container

Question

Dockerfile:

FROM python:3.10-slim
WORKDIR /watch
ADD requirements.txt /app/requirements.txt
RUN pip install --no-cache-dir -r /app/requirements.txt
ADD webwatcher /app/webwatcher

VOLUME /watch
ENV IS_DOCKER=true
ENV PYTHONPATH "/app"
ENTRYPOINT python3 -m webwatcher

My __main__.py in the module

import signal
from time import sleep
from .watchdog import register_observer

observer = register_observer()

def finish(signum, frame):
    print('\nExiting application...')
    observer.stop()
    observer.join()
    exit(0)

print('Registering signals')
signal.signal(signal.SIGTERM, finish)
signal.signal(signal.SIGINT, finish)

print('Running module')
observer.start()
while True:
    sleep(1)

I then build the container

docker build -t webwatcher:latest .

And run

docker run --rm --name web -v path:/watch/path webwatcher:latest

But when I run, it shows no output at all. It also does not let me quit with Ctrl+C from the run command, nor does docker stop work (it times out then kills it).

Why isn't the module being run properly from the container?

oartart · Accepted Answer · 2022-03-20 08:50:16Z

1

I think there is two separate issues here. By default, docker runs containers without TTY allocation, and some programs run slightly differently in those cases. Python will buffer stdout stream in this case. To prevent this, you can run docker run -it instead.

Another problem is that the Dockerfile has "shell-form" ENTRYPOINT python3 -m webwatcher instead of JSON notation ENTRYPOINT ["python3", "-m", "webwatcher"].

From hadolint docs:

The shell form prevents any CMD or run command line arguments from being used, but has the disadvantage that your ENTRYPOINT will be started as a subcommand of /bin/sh -c, which does not pass signals. This means that the executable will not be the container’s PID 1 - and will not receive Unix signals - so your executable will not receive a SIGTERM from docker stop.

If you enjoy using linters you might find hadolint useful to catch errors like this.

answered Mar 20, 2022 at 8:50

oartart

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1 Comment

cclloyd Over a year ago

When I tried using that form, it tells me that python isn't found. Searching online seemed to point towards using exec if that is an issue.

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