An option in some languages is to simply create an alternate mock/fake version of the actual class or function that is depended upon and monkey-patch the code under test to use it for the duration of the test. This is commonly done in Python ( see `unittest.mock.patch`) and JavaScript (eg. with Jest mocks) for example.

The end result is the same as if you'd created an interface or abstraction with two implementations (real and fake/mock), but you skip the part where a separate interface is defined.

The upside to this is that the code is more straightforward to follow. The downside is that the code is exposed/potentially coupled to the full API of the real implementation, as opposed to some interface exposing only what is needed.