Setting PYTHONUNBUFFERED to a non-empty value different from 0 ensures that the python output i.e. the stdout and stderr streams are sent straight to terminal (e.g. your container log) without being first buffered and that you can see the output of your application (e.g. django logs) in real time.
This also ensures that no partial output is held in a buffer somewhere and never written in case the python application crashes.
Since this has been mentioned in several comments and supplementary answers, note that PYTHONUNBUFFERED has absolutely no influence on the input (i.e. the stdin stream).
References:
- https://docs.python.org/3/using/cmdline.html#envvar-PYTHONUNBUFFERED
- https://alphacoder.xyz/dockerizing-django/
- https://towardsdatascience.com/how-to-contain-your-first-django-in-docker-and-access-it-from-aws-fdb0081bdf1d