Capturing output of python script run inside a docker container

Question

The aim here is to use a docker container as a secure sandbox to run untrusted python scripts in, but to do so from within python using the docker-py module, and be able to capture the output of that script.

I'm running a python script foo.py inside a docker container (it's set as the ENTRYPOINT command in my Dockerfile, so it's executed as soon as the container is run) and am unable to capture the output of that script. When I run the container via the normal CLI using

docker run -v /host_dirpath:/cont_dirpath my_image

(host_dirpath is the directory containing foo.py) I get the expected output of foo.py printed to stdout, which is just a dictionary of key-value pairs. However, I'm trying to do this from within python using the docker-py module, and somehow the script output is not being captured by the logs method. Here's the python code I'm using:

from docker import Client

docker = Client(base_url='unix://var/run/docker.sock',
              version='1.10',
              timeout=10)

contid = docker.create_container('my_image', volumes={"/cont_dirpath":""})
docker.start(contid, binds={"/host_dirpath": {"bind": "/cont_dirpath"} })

print "Docker logs: " + str(docker.logs(contid))

Which just results in "Docker logs: " - nothing is being captured in the logs, neither stdout nor stderr (I tried raising an exception inside foo.py to test this).

The results I'm after are calculated by foo.py and are currently just printed to stdout with a python print statement. How can I get this to be included in the docker container logs so I can read it from within python? Or capture this output some other way from outside the container?

Any help would be greatly appreciated. Thanks in advance!

EDIT:

Still no luck with docker-py, but it is working well when running the container with the normal CLI using subprocess.Popen - the output is indeed correctly grabbed by stdout when doing this.

Answer 1

You are experiencing this behavior because python buffers its outputs by default.

Take this example:

vagrant@docker:/vagrant/tmp$ cat foo.py
#!/usr/bin/python
from time import sleep

while True:
    print "f00"
    sleep(1)

then observing the logs from a container running as a daemon does not show anything:

vagrant@docker:/vagrant/tmp$ docker logs -f $(docker run -d -v $(pwd):/app dockerfile/python python /app/foo.py)

but if you disable the python buffered output with the -u command line parameter, everything shows up:

vagrant@docker:/vagrant/tmp$ docker logs -f $(docker run -d -v $(pwd):/app dockerfile/python python -u /app/foo.py)
f00
f00
f00
f00

You can also inject the PYTHONUNBUFFERED environment variable:

vagrant@docker:/vagrant/tmp$ docker logs -f $(docker run -d -v $(pwd):/app -e PYTHONUNBUFFERED=0 dockerfile/python python /app/foo.py)
f00
f00
f00
f00

Note that this behavior affects only containers running without the -t or --tty parameter.

Answer 2

You may experience a race condition since the started container is running in parallel to your control program. You need to wait for your container to start and finish before grabbing the logs. Add a docker.wait to your code right after docker.start.

docker.wait(contid)

Your output seems empty because nothing has yet been logged.