Warnings

The warnings module writes warning messages to the sys.stderr stream:

import warnings

warnings.warn("This is a drill!")

The output warning also contains a line number:

warn.py:3: UserWarning: This is a drill!

Python has an option to stop with an error as soon as a warning occurs:

python -W error warnme.py

You could also mute all warnings:

python -W ignore warnme.py