What? No. The author has a Duck, which has a quack() method. The quack() method is, presumably, identified by its name, argument types, and return type. Even in

What? No. The author has a Duck, which has a quack() method. The quack() method is, presumably, identified by its name, argument types, and return type. Even in a language with purely nominal typing, you would make a Quacker interface, which is the set of things that quack() -> None, and then have both Duck and all of its test stubs be Quackers. This is very literally how people who write in languages without structural types do test stubs (and often dependency injection, and other mechanisms that need to select from among several implementations of something).