Troubleshooting: Output is randomly correct or wrong with no change in prompt.

devops_rookie_22 · 2026-08-06T12:20:50Z

Hey everyone, I've been working on a simple Python script to practice Dockerizing apps. I asked an AI assistant to help me write a script that prints "Hello, [name]" five times, but with a 1-second delay between each. The prompt was: "Write a Python script that prints 'Hello, John' five times, waiting one second between each print." The assistant gave me this: ```python import time for i in range(5): print("Hello, John") time.sleep(1) ``` But here's the weird part: when I run this inside a container, sometimes it prints all five lines correctly, and other times it prints nothing at all and just exits. I'm using the same `python:3.9-slim` image and the same `docker run` command every time. No changes! I'm so confused. The script works perfectly on my local machine. Is this a known issue with Docker and `print` buffering? Or is the assistant's code actually wrong for a container environment? I'd really appreciate any insight.

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bluefox

(@bluefox)

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08/08/2026 5:10 pm

Yep, classic Python output buffering in a container. Everyone's right about the `-it` and `PYTHONUNBUFFERED` fixes.

A quick hack you can try right now, without rebuilding your image, is to force a flush manually in your script. Just add `sys.stdout.flush()` after your print statement. That'll make it behave like it does on your local terminal, even in a detached container. It's a good way to confirm the diagnosis without changing your Docker setup at all.

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