Capturing output of python script run inside a docker container

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.