Why Doesn\'t My Detached Docker Container Show Python Print Output?

Detached Docker Containers and Print Output in Python Apps

When running Python (2.7) applications in a Docker container detached (using the -d flag), users may encounter a situation where the application's print statements don't produce any output.

Understanding Python Output Buffering

Python uses buffered output by default, meaning it stores output in a buffer before sending it to the screen. In a detached container, this buffer may not be flushed frequently enough, leading to the absence of output.

Using Unbuffered Output

To resolve this issue, unbuffered output can be used with the -u flag:

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

This ensures that output is written to the screen immediately, regardless of the buffering settings.

Docker Log Correlation

With unbuffered output, print statements will be visible in the Docker logs:

docker logs myapp
> App started

Detached Container Behavior without "-u"

When the -u flag is not used, print behaves as follows in a detached container:

• Output is stored in a buffer.
• If the buffer becomes full, it is flushed to the screen.
• If the container exits before the buffer is full, the output is lost.

Debugging Tips

• Use python -u flag: Test the application with unbuffered output outside Docker to ensure that the issue is container-specific.
• Check docker logs: Even without the -u flag, output should be visible in the Docker logs, albeit delayed.
• Attach to container: Attaching to the container with docker attach may display output if the buffer is small enough to be flushed before detaching.

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