The Three Laws of Test-Driven Development

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Revision as of 13:16, 26 November 2008

The jury is in. The controversy is over. The debate has ended, and the conclusion is: TDD works. Sorry.

Test-Driven Development (TDD) is a programming discipline whereby programmers drive the design and implementation of their code by using unit tests. There are three simple laws:

  1. You can't write any production code until you have first written a failing unit test.
  2. You can't write more of a unit test than is sufficient to fail, and not compiling is failing.
  3. You can't write more production code than is sufficient to pass the currently failing unit test.

If you follow these three laws you will be locked into a cycle that is, perhaps, 30 seconds long.

Upon first hearing these laws, experienced programmers conclude that they are just stupid. But if we work this way, we soon discover that there are a number of benefits:

  • Debugging
    What would programming be like if you were never more than a few minutes away from running everything and seeing it work? Imagine working on a project where you never have several modules torn to shreds hoping you can get them all put back together next Tuesday. Imagine your debug time shrinking to the extent that you lose the muscle memory for your debugging hot keys.
  • Courage
    Have you ever seen code in a module that was so ugly that your first reaction was "Wow, I should clean this." Of course your next reaction was: "I'm not touching it!". Why? Because you were afraid you'd break it. How much code could be cleaned if we could conquer the fear that we'd break the code we were cleaning? If you have have a suite of tests that you trust, then you are not afraid to make changes.
    You are not afraid to make changes!
    You see a messy functions, you clean it. All the tests pass! The tests give you the courage to clean the code!
    Nothing make a system more flexible than a suite of tests — nothing. If you have a beautiful design and architecture, but have no tests, you are still afraid to change the code. But if you have a suite of high-coverage tests, then even if your design and architecture are terrible, you are not afraid to clean up the design and architecture.
  • Documentation
    Have you ever integrated a third party package? You get a nice manual written by a tech writer. At the back of that manual is an ugly section where all the code examples are shown. Where's the first place you go? You go to the code examples. You go to the code examples because that's where the truth is.
    Unit tests are just like those code examples. If you want to know how to call a method, there are tests that call that method every way it can be called. These tests are small, focused documents that describe how to use the production code. They are design documents that are written in a language that the programmers understand; are unambiguous; are so formal that they execute; and cannot get out of sync with the production code.
  • Design
    If you follow the three laws every module will be testable by definition. And another word for testable is decoupled. In order to write your tests first, you have to decouple the units you are testing from the rest of the system. There's simply no other way to do it. This means your designs are more flexible, more maintainable, and just cleaner.
  • Professionalism
    Given that these benefits are real, the bottom line is that it would be unprofessional not to adopt the practice that yields them.

by Uncle Bob

This work is licensed under a Creative Commons Attribution 3

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