How to use locally built package?

I am working on two Python packages. One of which (let's call it B) uses the other (let's call the A). 

I would like to be able to make changes in A which are then used when running and debugging B. 

What is the best way to do this? 

I have tried adding a path to the interpreter in use for package B, but don't know which directory to point it at within the structure of package A. And if I have to go and manually rebuild A for every change, which seems like a pain. 

(In IntelliJ one would add a separate module to the project, but I don't see how to do that here.)

thanks, Peter