How to run python script before any stack updates using Sceptre

Question

I have a Python script that I'd like to run before any stack is updated and deployed. From my research, I know I need to use hooks and likely the hook point before_update.

What I can't find is the syntax for running a python script from the hook, and how to ensure that the script runs every time ANY of my stacks are updated (ie. I don't want to have to write the hook for every stack, but have this hook set globally instead if possible).

I'm assuming the place to write the hook would be in cloudformation\config\config.yaml but would love confirmation of this as well :)