Test Driven Development - Deep Dive

Test Driven Development (TDD) - Deep Dive

Test-driven development (TDD), is an evolutionary approach to development which combines test-first development where you write a test before you write just enough production code to fulfill that test and refactoring.   What is the primary goal of TDD? One view is the goal of TDD is specification and not validation. In other words, it’s one way to think through your requirements or design before your write your functional code (implying that TDD is both an important agile requirements and agile design technique).  Another view is that TDD is a programming technique.  The goal of TDD is to write clean code that works.  I think that there is merit in both arguments, although I lean towards the specification view, but I leave it for you to decide.

The first step is to quickly add a test, basically just enough code to fail.  Next you run your tests, often the complete test suite although for sake of speed you may decide to run only a subset, to ensure that the new test does in fact fail.  You then update your functional code to make it pass the new tests.  The fourth step is to run your tests again.  If they fail you need to update your functional code and retest.  Once the tests pass the next step is to start over (you may first need to refactor any duplication out of your design as needed, turning Test First Development (TFD) into TDD).

TDD completely turns traditional development around. When you first go to implement a new feature, the first question that you ask is whether the existing design is the best design possible that enables you to implement that functionality.  If so, you proceed via a TFD approach.  If not, you refactor it locally to change the portion of the design affected by the new feature, enabling you to add that feature as easy as possible.  As a result you will always be improving the quality of your design, thereby making it easier to work with in the future.

Instead of writing functional code first and then your testing code as an afterthought, if you write it at all, you instead write your test code before your functional code.   Furthermore, you do so in very small steps – one test and a small bit of corresponding functional code at a time.   A programmer taking a TDD approach refuses to write a new function until there is first a test that fails because that function isn’t present.  In fact, they refuse to add even a single line of code until a test exists for it.  Once the test is in place they then do the work required to ensure that the test suite now passes (your new code may break several existing tests as well as the new one).   This sounds simple in principle, but when you are first learning to take a TDD approach it proves require great discipline because it is easy to “slip” and write functional code without first writing a new test.  One of the advantages of pair programming is that your pair helps you to stay on track.

There are two levels of TDD:

1. Acceptance TDD (ATDD).  With ATDD you write a single acceptance test, or behavioral specification depending on your preferred terminology, and then just enough production functionality/code to fulfill that test.  The goal of ATDD is to specify detailed, executable requirements for your solution on a just in time (JIT) basis. ATDD is also called Behavior Driven Development (BDD).
   
2. Developer TDD. With developer TDD you write a single developer test, sometimes inaccurately referred to as a unit test, and then just enough production code to fulfill that test.  The goal of developer TDD is to specify a detailed, executable design for your solution on a JIT basis.  Developer TDD is often simply called TDD.

Ideally, you'll write a single acceptance test, then to implement the production code required to fulfill that test you'll take a developer TDD approach. This in turn requires you to iterate several times through the write a test, write production code, get it working cycle at the developer TDD level.

Why TDD?

A significant advantage of TDD is that it enables you to take small steps when writing software.  This is a practice that I have promoted for years because it is far more productive than attempting to code in large steps.  For example, assume you add some new functional code, compile, and test it.  Chances are pretty good that your tests will be broken by defects that exist in the new code.  It is much easier to find, and then fix, those defects if you've written two new lines of code than two thousand. The implication is that the faster your compiler and regression test suite, the more attractive it is to proceed in smaller and smaller steps.  I generally prefer to add a few new lines of functional code, typically less than ten, before I recompile and rerun my tests.


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