How to fix the code change during parsing?

Question

Python

Task: Fix changes when adding or removing elements on the page (in my case, a new ad on a large number of specialized sites).
I'm new, tell me where to look. I understand that in order to fix the change, you need to constantly parse the site (maybe every 30 seconds) and compare with previous indicators. I would like to understand the algorithm and a simple example for a couple of lines. What libraries and methods.
Thank you.

Answer 1

1. Understand what ads are present on the page (first of all, pull out their unique IDs)
   
2. Retrieve the previous version of the page from the database
   
3. Build diff .