A better thing to do is do phased deployment, so you can see if an update will cause issues in your environment before pushing it to all systems. As this incident shows, you can’t trust a software vendor to have done that themselves.
I've mostly resigned myself today to deploying the configuration change and watching for anomalies in my monitoring for a number of hours or days afterward, but I acknowledge that I also have both a process supervisor that will happily let me crash loop my programs and deployment infrastructure that will nonetheless allow me to roll things back. Without either of those, I'm honestly at a loss as to how I'd safely operate this product.
# Update A
## config.ext
foo = false
## src.py
from config import config
if config('foo'):
work(2 / 0)
else:
work(10 / 5)
# Update B
## config.ext
foo = true
The most insidious part of this is when there are entire swaths of infrastructure in place that circumvent the usual code review process in order to execute those configuration changes. Boolean flags like your `config('foo')` here are most common, but I've also seen nested dictionaries shoved through this way.