Test isolation is a key element of any test automation strategy with unit and integration tests. The AgileSE course teaches how to do test isolation in the OO (ABAP Objects) context via classes, interfaces/inheritance and dependency injection. All this is quite natural in the context of OO code and fits to the general rules of ‘good OO design’, decoupling etc. However, for people not working in OO all the time the patterns can be challenging and make test isolation seem as ‘too hard’.
For code that is completely functional / non-OO and developers that work mostly with non-OO code, there is a simple solution for test isolation for FUNCTIONs and FORMs that allows you to do unit testing very easily, also in functional code. The solution is described here: https://scn.sap.com/docs/DOC-45121.