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

Since I haven't seen this answer yet:

You can also flush stdout after you print to it:

import time

if __name__ == '__main__':
    while True:
        print('cleaner is up', flush=True)
        time.sleep(5)

Answer 2

I had to use PYTHONUNBUFFERED=1 in my docker-compose.yml file to see the output from django runserver.

Answer 3

See this article which explain detail reason for the behavior:

There are typically three modes for buffering:

• If a file descriptor is unbuffered then no buffering occurs whatsoever, and function calls that read or write data occur immediately (and will block).
• If a file descriptor is fully-buffered then a fixed-size buffer is used, and read or write calls simply read or write from the buffer. The buffer isn’t flushed until it fills up.
• If a file descriptor is line-buffered then the buffering waits until it sees a newline character. So data will buffer and buffer until a \n is seen, and then all of the data that buffered is flushed at that point in time. In reality there’s typically a maximum size on the buffer (just as in the fully-buffered case), so the rule is actually more like “buffer until a newline character is seen or 4096 bytes of data are encountered, whichever occurs first”.

And GNU libc (glibc) uses the following rules for buffering:

Stream               Type          Behavior
stdin                input         line-buffered
stdout (TTY)         output        line-buffered
stdout (not a TTY)   output        fully-buffered
stderr               output        unbuffered

So, if use -t, from docker document, it will allocate a pseudo-tty, then stdout becomes line-buffered, thus docker run --name=myapp -it myappimage could see the one-line output.

And, if just use -d, no tty was allocated, then, stdout is fully-buffered, one line App started surely not able to flush the buffer.

Then, use -dt to make stdout line buffered or add -u in python to flush the buffer is the way to fix it.

Answer 4

If you want to add your print output to your Flask output when running docker-compose up, add the following to your docker compose file.

web:
  environment:
    - PYTHONUNBUFFERED=1

https://docs.docker.com/compose/environment-variables/

Answer 5

Try to add these two environment variables to your solution PYTHONUNBUFFERED=1 and PYTHONIOENCODING=UTF-8

Answer 6

If you aren't using docker-compose and just normal docker instead, you can add this to your Dockerfile that is hosting a flask app

ARG FLASK_ENV="production"
ENV FLASK_ENV="${FLASK_ENV}" \
    PYTHONUNBUFFERED="true"

CMD [ "flask", "run" ]