It's supposed to be this way but in practice it doesn't seem to work. With methods/functions it's a problem when they call other functions. So really the idea

It's supposed to be this way but in practice it doesn't seem to work. With methods/functions it's a problem when they call other functions. So really the idea that a good design is also testable is somewhat of an approximation of reality, It has some elements of truth but at some point making your code more testable actually makes it worse, often you need to make other compromises. Then the interactions of pieces of code typically happens in much more brittle system/integration tests. YMMV, this is just my experience. A lot of interesting pointers in the replies, thanks!