Creating partial mocks that fake out a subset of methods

If you've looked at Mocktail's APIs for creating mock instances and replacing class or module methods, you'll find that it's an a bit of an all or nothing affair. You can get create an instance of a class with all its methods replaced with nil-returning fakes and you can replace all a module's module methods, but you can't tell Mocktail to only replace one or a handful of methods while continuing to call through to some other real methods.

Why?! Why would we do something so unfair as to withhold such obviously useful functionality?

Well, the answer—like everything having to do with test doubles—requires some nuance. In testing parlance, to replace some of the methods on a dependency but not all of them is to create what is called a partial mock (click through its glossary definition for some of the reasons partial mocks are considered to be an antipattern).

Mocktail was written to promote test-driven development that specifies thoughtfully-designed classes that interact with other classes, but partial mocks actually detract from that purpose in practice. As a result Mocktail, doesn't offer a way to create partial mocks.

If you find yourself wanting to reach for a partial mock, we'd encourage you to first take it as potential design feedback that a dependency is perhaps too big and its contract with subject too porous.

If you've heard enough, you can go back and consider non-TDD use cases for Mocktail.

Or if you're finally ready to walk the golden path, you can revisit Mocktail as a TDD tool.