This is something I've been grappling with as well. My current 'solution' is to have separate notebooks with a function in each one (organized in a 'Functions' directory). Then I load the function into memory in another notebook by loading that notebook with single function.

It's not very elegant but at least the function called exists only in a single location.

I'd like to know if there is better way to do this. My current implementation is workable but far from the ideal.

Edit: this is the forum question/answer concerning the loading of a notebook within a notebook: https://forums.databricks.com/questions/154/can-i-run-one-notebook-from-another-notebook.html