Test-Driven Development

Steven J Zeil

Last modified: Dec 21, 2019


Test-Driven Development treats unit testing as an integral part of the design and implementation process. Often summarized as “test first, code after”, TDD is actually a recognition that in writing tests, we are

1 Test-First Development

With our new knowledge of unit-testing frameworks, ideally, we have made it easier to write self-checking unit tests than to write the actual code to be tested.

1.1 Debugging: How Can You Fix What You Can’t See?

The test-first philosophy is easiest to understand in a maintenance/debugging context.

1.2 Test-Writing as a Design Activity

Every few years, software designers rediscover the principle of writing tests before implementing code.

Agile and TDD (Test-Driven Development) are just the latest in this long chain.

1.2.1 Tests are Examples

“If it’s hard to write a test, it’s a signal that you have a design problem, not a testing problem. Loosely coupled, highly cohesive code is easy to test.” – Kent Beck

1.3 The Cycle of Unit Test Failures


Here you can see a plot of test cases on the vertical axis versus time (actually, commits to the version control system) on the horizontal axis during a project on which I practiced TFD.

Tests passed are shown in blue and failed tests are shown in red.

Notice the repeated pattern:

2 TFD during Incremental Development

My stereotypical division of a story into tasks is typically

  1. Create/modify the API to describe a new desired behavior.
  2. Write the unit tests.
  3. Implement the new behavior.
  4. Integrate and commit changes.

Compare this to the steps of TDD, above, and you can see that they are compatible.

2.1 Case Study: TFD of a Spreadsheet Story

Here are some short videos illustrating my application of task 1 and task 2 of a story for the Embeddable Spreadsheet project.

3 Test-Driven Development

Test-Driven Development (TDD) is a stronger form of Test-First Development.

In TDD, we repeatedly:

  1. Write an automated test case for a new desired behavior.

    • This case must, initially, fail.
    • Not compiling counts as “failing”.
  2. Write just enough new code to pass the test.

  3. Refactor the code to make it acceptable quality.

This ties in very nicely with some of our previous discussion of incremental development. In particular, compare to the way we break stories into tasks.

3.1 The “Three Rules of TDD”

From Robert Martin

Over the years I have come to describe Test Driven Development in terms of three simple rules. They are:

  1. You are not allowed to write any production code unless it is to make a failing unit test pass.
  2. You are not allowed to write any more of a unit test than is sufficient to fail; and compilation failures are failures.
  3. You are not allowed to write any more production code than is sufficient to pass the one failing unit test.

You must begin by writing a unit test for the functionality that you intend to write. But by rule 2, you can’t write very much of that unit test. As soon as the unit test code fails to compile, or fails an assertion, you must stop and write production code. But by rule 3 you can only write the production code that makes the test compile or pass, and no more.

If you think about this you will realize that you simply cannot write very much code at all without compiling and executing something. Indeed, this is really the point. In everything we do, whether writing tests, writing production code, or refactoring, we keep the system executing at all times. The time between running tests is on the order of seconds, or minutes. Even 10 minutes is too long.

3.2 Example of TDD

The Bowling Game Kata (from Martin.)