But don't those exist primarily for unit testing? That was my understanding of why interfaces were there.
If you wanted a mock, you needed an interface, even if there would ever only be one implementation of it in production.
If you wanted a mock, you needed an interface, even if there would ever only be one implementation of it in production.
I believe that's why people insert them everywhere, yes, but in the codebases I'm talking about, many (I'd say the majority, to be honest) of the interfaces aren't used for testing because they've just been cargo-culted rather than actually considered.
(Obviously this is with hindsight - they may well have been considered at the time but the end result doesn't reflect that.)
You need to refactor it to use an interface just to unit test it.
Duck typing with python (e.g.) makes it easy to mock a class. One can subclass a Java class to make a mock, etc.
One can't do that in golang unless it's through an interface which then decouples tightly coupled code because [golang reasons].