[daleharvey]

Testing dependencies is not a bug, if there is a reason to not test them, like you need to test an error condition, or your dependancy is external (oauth etc), then certainly, but if there is no need to mock a dependency other than a dogma of some definition of unit test, then it usually isnt worth it.

With every test the questions should be answered are what bugs is this going to catch and which one will it miss, if you mock a dependency then you are introducing cases in which it will miss bugs and there should be a justification along with it.

[mbrock]

The classic justification is that mocking encourages modular code where each unit has shallow dependencies with well-defined interfaces.

This also means that a mistaken change to one important unit will not break the entire test suite. Sure, the entire program will break, but it's nice to get a single failing test.

Mocking also gives a very straightforward way to simulate interactions with collaborators. You just say "given that the HTTP module returned a 404, this request should be sent to the error log," instead of initializing those two modules and arranging for the desired case to happen.

There's a very old discussion about decreasing coupling and increasing cohesion that's super important to the whole motivation behind TDD and that nobody seems to be very interested in anymore...