Test Driven Interviewing

For most¹ engineering tasks, I prefer to avoid TDD, or Test Driven Development.

Yet when I job-hunt, I use TDD religiously². It has been tremendously helpful.

Hmpf. Why?

Most interview challenges actually come in two parts.

1. Can you take a broadly-worded problem and nail it down into something unambiguous?
2. Can you implement a challenging but unambiguous spec in the allotted time?

TDD forces you into the ideal mindset for nailing down (1) problem definitions. There’s no better way to properly grok a problem than to have to think through all the fun ways an implementation could be slightly off.

The implementation itself (2) also gets easier since you no longer have to assume that your code “probably” works, or fiddle with a REPL each time you’re ready to check.

So how do I do it?

As soon as you hear the problem, resist the urge to rush straight to the solution.

Instead, force yourself to fully think through the test cases for an arbitrary implementation. Make it a dialog - “what if the same element is in the array multiple times?” Get the interviewer engaged. Show you care.

By the time you’ve written good tests and the interviewer agrees, you’re all set. Even though you tried not to, you probably already have a decent solution in mind. Your tests will let you know you’re done. They’ll also give you the comfort that your subsequent clean up / refactoring doesn’t inadvertantly break your implemenation.

Yeah, but I can always just write tests later.

When I conduct interviews, I keep running into candidates clever enough to power through engineering challenges on raw talent³. The candidate implements an optimal solution within fifteen minutes. Then it’s time for tests and she just cannot think of cases beyond the common path, which makes her look like a cowboy.

I blame the curse of knowledge - once you’ve solved the problem, it’s harder to think about what wrong turns you could have taken. It all feels trivial.

Seriously though, are you going to make me import a testing framework? Like, Mocha or Cucumber? Ugh.

¯\_(ツ)_/¯, I just use asserts and print statements. If I’m feeling fancy, I’ll create an array of test case scenarios and run them like

    test_cases = [
      [[2, 2], 4],
      [[1, 2, 3, 4], 10],
      [[], 0],
      [[-3, 2], 1],
      [[100, 2], 102],
      [["banana"], None]
    ]

    for test_case in test_cases:
      args, expected_answer = test_case
      answer = quantum_addition(args)

      print("called add({}) and got {}, expected {}"
        .format(args, answer, expected_answer))

      if answer != expected_answer:
        print("UH OH!")

  # (or you could just do: assert answer == expected_answer)

If you insist on being fancy, you can do that thing where you ask your interviewer if they’d like you to use a proper testing framework first, and of course they don’t, but now it looks like you could and you’re just doing them a favor by using asserts, cause you’re like, super assertive.

Ok so now what?

Well obviously you should come work at Opendoor with me! We have the good coconut water and don’t run out of it until like noon, 12:30 on good days. Plus, there’s a decent chance you’ll run into me on your phone screen and we can have that awkward interaction of trying to figure out why my name sounds familiar.

Thanks to Vaibhav, Joe, Nick, Gayle, Tess, Charlie, Kevin, Zain, and Yahel for giving feedback on earlier drafts that I incorporated.

Tags: #technical-interviews #advice-for-engineers #engineering-management