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

Understanding Print Behavior in Detached Docker Python Containers

In Python applications running within detached Docker containers, you may encounter a lack of output when using the print statement. This issue arises when the STDOUT and STDERR streams are buffered within the container.

The Problem

When you execute a Python app (2.7) using Docker's CMD ["python","main.py"], the main.py script initializes with print "App started" and enters an indefinite loop. Observing the container with the -it flag shows the expected output in both the terminal and Docker logs.

However, using the -d flag (detached mode) launches the container but suppresses the expected output from being visible in the logs or terminal.

The Solution

To resolve this behavior, Python's unbuffered output option can be utilized. By modifying the Dockerfile CMD to CMD ["python","-u","main.py"], the buffering of STDOUT and STDERR is disabled. This allows Python to print immediately, making the output visible when querying docker logs myapp.

Why Unbuffered Output?

The -u argument forces Python to flush output to the standard streams without waiting for a buffer to fill. This ensures that prints are displayed promptly, even when the script is running in the background.

Additional Observations

In contrast to print, using the logging module's logging.warning("text") function generates immediate output even without unbuffered mode. This is because logging handles flushing internally.

The Python documentation for -u option states that it unbuffers both STDOUT and STDERR streams, allowing for live output display in a detached container.

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