Explore the concept of the hooks.
override js: Hooks
for py:
- Override Whitelisted Methods
- Override DocType Class
- Also you check the all videos in YouTube
- Override Whitelisted Methods - Part 1
- Override Doctype Class-Based Whitelisted Method - Part 2
- Override Doctype Class Method - Part 3 (However, we do not recommend overriding the doctype class method or using monkey patches. Instead, a better solution is to use doc_events for this purpose.)
I hope this helps.