TIL: Python Output Buffering

I created a simple Python service, and was manually monitoring it using logs. I had both a FileHandler and a StreamHandler(sys.stdout). Later I had to Dockerize the service, mostly as a not-well-thought-out way to manage Python versions. I hit my first hitch when I discovered it's too much trouble to use Alpine Linux for Python builds. When that was resolved by switching to Debian, the logs took forever to show up and docker compose logs would yield nothing for a while.

A quick search ^[1] ^[2] showed that Python buffers output such as stdout and when writing to a file for performance reasons. So the logging or print statements would not show up until the buffer was full. My solution was to simply add the -u flag to the script invokation: python -u main.py. A few other solutions have been offered on StackOverflow ^[1:1] such as the PYTHONUNBUFFERED=1 environment variable. This "issue" showed up when running a daemonized Docker container (read docker compose up -d) since Python detects if a tty is attached and disables block buffering*. Tangetially, I do remember using the flush=True keyword argument with print some time back, but I can't remember why I used it.