48-Hour Debugging Mystery Solved: Python Buffers stdout When No TTY Is Attached

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A developer spent nearly 48 hours troubleshooting what appeared to be a deadlocked remote batch job, only to discover the real culprit was Python's default stdout buffering behavior. CPython automatically switches to block-buffering mode when stdout is not connected to a terminal (TTY), such as when output is captured via a pipe on a remote server. This caused all print statements to be silently held in a buffer and only flushed when the process crashed, making the job appear frozen. The issue did not reproduce locally because a laptop terminal counts as a TTY, which triggers line-buffering and immediate output. The fix is straightforward: run Python with the -u flag or set the PYTHONUNBUFFERED=1 environment variable to force unbuffered stdout in non-TTY environments.

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