Python app does not print anything when running detached in docker

Finally I found a solution to see Python output when running daemonized in Docker, thanks to @ahmetalpbalkan over at GitHub. Answering it here myself for further reference :

Using unbuffered output with

CMD ["python","-u","main.py"]

instead of

CMD ["python","main.py"]

solves the problem; you can see the output (both, stderr and stdout) via

docker logs myapp

now!

In my case, running Python with -u didn't change anything. What did the trick, however, was to set PYTHONUNBUFFERED=1 as environment variable:

docker run --name=myapp -e PYTHONUNBUFFERED=1 -d myappimage

[Edit]: Updated PYTHONUNBUFFERED=0 to PYTHONUNBUFFERED=1 after Lars's comment. This doesn't change the behavior and adds clarity.