Part ten of a series of posts about automated testing.

While discussing unit testing, it is worth looking at the idea of test-driven development, and how we might use it to improve the quality of our code.

The TDD approach goes:

  1. Write one failing test (“Red”)
  2. Write the smallest change that makes all tests pass (“Green”)
  3. Refactor the code to remove duplication etc. (“Refactor”)

Repeat until done.

The devil is perhaps in the detail. TDD demands small increments of code - it can be tempting to jump ahead. For example, given this test case:

sub score_should_be_zero : Test {
is(score(), 0);

If score() has not been written, the test doesn’t compile, so it can’t be “red”. The test has to compile and fail, so the next step is:

sub score {
return -1;

Note that we do not return zero yet! This is to ensure that the test is actually testing the right thing - if you get a green bar at this point, you can spot something is wrong. Finally you correct the implementation to match the test:

sub score {
return 0;

The recommended way to get into TDD is to practice TDD kata - short exercises which you repeat daily until you have mastered them, then you move on to the next one. The example above is adapted from the bowling game kata, which comes with a worked example. Search online for more.

TDD is great when writing code from scratch, but it can be more difficult to apply to legacy code, because:

  • if you don’t already have a good test suite, it is hard to refactor correctly.
  • often it is hard to isolate external dependencies in order to unit test code, unless the code has been written with testing in mind.

Therefore in this series we will move on to consider how to write code which is easy to test.