Python app does not print anything when running detached in docker

Question

I have a Python (2.7) app which is started in my dockerfile:

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

main.py prints some strings when it is s

Answer 1

You can see logs on detached image if you change print to logging.

main.py:

import time
import logging
print "App started"
logging.warning("Log app started")
while True:
    time.sleep(1)

Dockerfile:

FROM python:2.7-stretch
ADD . /app
WORKDIR /app
CMD ["python","main.py"]

Answer 2

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!

Answer 3

As a quick fix, try this:

from __future__ import print_function
# some code
print("App started", file=sys.stderr)

This works for me when I encounter the same problems. But, to be honest, I don't know why does this error happen.

Answer 4

Usually, we redirect it to a specific file (by mounting a volume from host and writing it to that file).

Adding a tty using -t is also fine. You need to pick it up in docker logs.

Using large log outputs, I did not have any issue with buffer storing all without putting it in dockers log.

Answer 5

In my case, running Python with -u didn't change anything. What did the trick, however, was to set PYTHONUNBUFFERED=0 as environment variable:

docker run --name=myapp -e PYTHONUNBUFFERED=0 -d myappimage