Why Doesn\'t My Python App Print Anything When Run in a Detached Docker Container?

Why Python App Prints Nothing in Detached Docker Container

When executing a Python app within a Docker container in detached mode (-d flag), you may encounter an issue where the app's output is not visible.

Reason

Python's "print" function uses buffered output, which means the output is not immediately flushed to the stdout stream. When running in detached mode, the container process exits after the main script (main.py) completes its loop. Since the stdout buffer has not been flushed yet, the output is lost.

Solution

To resolve the issue, enable unbuffered output using the -u flag with the Python command in the Dockerfile's CMD instruction:

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

With unbuffered output, the app's output will be immediately written to stdout, making it visible when using docker logs even in detached mode:

$ docker run --name=myapp -d myappimage
> b82db1120fee5f92c80000f30f6bdc84e068bafa32738ab7adb47e641b19b4d1

$ docker logs myapp
> App started
> [loop output]

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