Selecting and ordering test cases

FizzBuzz is one step up in difficulty from Leap Years. The problem description asks you to design a console application. You need to do a small amount of analysis to realize the heart of it is a pure function similar to the one for Leap Years. This one takes an integer and returns a String. You can use this Kata to talk about how to plan and select test cases, and why you spend a few minutes analyzing the problem to look for a pure function to test drive. Note - this is part of the general strategy for ‘inside-out’ TDD.

Learning goals

The theme is the same as the first session: “Small steps”.

Session Outline


In pairs discuss this question.

Which is easiest to write automated tests for:

(Use the internet to research if you are unsure what those things are.)

The reason for asking about what is easy to test is to get them prepared for designing a pure function or static method for calculating FizzBuzz. These kinds of functions are easier to test since all the outcomes are visible in the return value, the input value is not modified and there are no side effects.

Explain and elaborate test cases

Starting at a whiteboard, explain the purpose of the FizzBuzz kata. Read the description to the group, or put it on a screen they can see. Ask the group to suggest test cases we will need for this kata. Write up whatever they say.

What they come up with might include this - an impure function that prints a multi-line string:

() -> print("1\n2\nFizz\n4\nBuzz\n...")

Explain that they could usefully write a pure function that takes an integer and returns a string. This function could be called by the other one, and will be easier to test, while still giving us good coverage of the business rules.

You might then end up with test cases on the whiteboard looking something like this:

1 -> "1"
3 -> "Fizz"
5 -> "Buzz"
6 -> "Fizz"
15 -> "FizzBuzz"

When you have half a dozen or so of those kinds of cases, ask them to prioritize/sort them. Which is the easiest to implement? Which order should we take them in? Hopefully they will notice you should take a number that is not divisible by 3 or 5 to begin with. Write numbers next to the examples (in a different colour pen) which order to implement them as test cases.

Inside-out vs Outside-In TDD

Talk about how you plan and select test cases in TDD. Point out why you might spend a few minutes analyzing the problem to look for a pure function to test drive. Explain this is a strategy for ‘inside-out’ TDD, and explain what that is. I like to make a sketch as I talk or have a couple of slides prepared. Share a link to more information (either something you wrote or an article or video or book that you recommend).


Have the group work in pairs or ensemble to do the Kata, starting from no code, just the examples written on the whiteboard. Every 4 minutes, remind them to swap the typist.

When it comes to the test for the impure function that prints to the console, I suggest you point out this is harder to test with a unit test and we will leave automating that test for another day. For today it’s enough to automate the test for the pure function, and do a manual test of printing to the console.


Take the ‘group temperature’ on these questions - ask them to rate their confidence on a scale of 1-5

Since they have probably not written an automated test for the overall solution that prints all the FizzBuzz numbers up to 100, they might feel less confident in that part of the code. On the other hand, they didn’t need any mocks or test doubles in the tests they wrote.

In pairs, ask them to discuss: Can you think of other code you’ve written where you could have test-driven the ‘inside’ part?