The warnings module writes warning messages to the sys.stderr stream:
:::python
import warnings
warnings.warn("This is a drill!")
The output warning also contains a line number:
:::bash
warn.py:3: UserWarning: This is a drill!
Python has an option to stop with an error as soon as a warning occurs:
:::bash
python -W error warnme.py
You could also mute all warnings:
:::bash
python -W ignore warnme.py