Unit testing Go code with mocks and dependency injection

Not continuously testing code is no longer an option. If you don't test, you don't know if new features actually work – or if a change adds new bugs or even tears open old wounds again. While the Go compiler will complain about type errors faster than scripting languages usually do, and strict type checking rules out whole legions of careless mistakes from the outset, static checks can never guarantee that a program will run smoothly. To do that, you need a test suite that exposes the code to real-world conditions and sees whether it behaves as expected at run time.

Ideally, the test suite should run at lightning speed so that developers don't get tired of kicking it off over and over again. And it should be resilient, continuing to run even while the Internet connection on the bus ride to work occasionally drops. So, if the tests open a connection to a web server or need a running database, this is very much out of line with the idea of fast independent tests.

However, since hardly any serious software just keeps chugging along by itself without a surrounding infrastructure, it is important for the test suite to take care of any dependencies on external systems and replace them with Potemkin villages. These simulators (aka "mocks") slip into the role of genuine communication partners for the test suite, accepting its requests and returning programmed responses, just as their real world counterparts would.

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