Why Do My Dockerized Python Apps Show No Output When Run Detached?

Dockerized Python Apps: Why No Output with Detached Execution?

Background

When executing Python scripts in Docker containers, output behavior can differ depending on whether the container is run in interactive or detached mode.

Issue

In detached mode (-d flag), Python scripts may not print any output, even though the container appears to be running normally. This issue affects Python (version 2.7) apps started with CMD ["python", "main.py"] in their Dockerfile.

Cause

The problem stems from Python's default buffered output behavior. When running detached, the container's output buffers are not flushed automatically, so they accumulate until a certain threshold is reached. This results in the absence of visible output during detachment.

Solution

To resolve this issue, use unbuffered output (-u flag):

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

This forces stdout and stderr to be unbuffered, allowing output to be displayed immediately in detached containers.

Explanation

• Buffered Output: By default, Python buffers stdout and stderr before flushing them.
• Detached Mode: In detached mode, this buffering prevents immediate output, as the container is not actively monitored for output.
• Unbuffered Output: The -u flag disables buffering, ensuring instant output display.

By enabling unbuffered output, the container's output can be viewed in real-time during detached execution.

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