I don't really get what testing has to do with whether or not a piece of code is isolated or not? For example if you're making a change to a 4 line function wit

I don't really get what testing has to do with whether or not a piece of code is isolated or not? For example if you're making a change to a 4 line function with a clearly defined interface, the change is probably going to be trivial, regardless of whether the interface is defined by a set of unit tests or a short comment above the function declaration