# qiBuild documentation

# qibuild: using test driven development

## Red, Green, Refactor

Test Driven Development is an old technique, but we found it very effective during the development of qibuild.

To be truly effective, your code must go through three steps: red, green, refactor

Assuming you want to fix a bug.

First, you write a failing test reproducing the bug.

Then, you change the code until the test passes.

Then, you refactor the code if you need too, making sure that all test pass.

## Rationale

### Write a failing test first

Having a test that fails at first is a good thing, because you can know the test is actually testing something, and is not doing a sophisticated version of

    def test_foo(self):
       self.assertTrue(1 == 1)

In case of adding a new features, writing the test first forces you to think about the testability of the new code you are going to write, which may seem a waste of time.

But code that is easy to test is easy to change or refactor (see below), and the fact that the code is testable usually means there is a good decoupling of concerns, which is also a good thing.

Writing the tests first also makes you think about the specifications of your new features. If you do not have a clear set of specifications in mind, your are not going to write good software.

Note that it is perfectly reasonable to also add the specifications in the qibuild documentation even before writing the tests, that’s a good way to get feedback. It may seem strange to start by writing the documentation, but we also find this technique very effective.

Last, but not least, writing the tests first makes you think about the interface of the new code you are going to write, which is also a good thing: clear and clean API also make for more maintainable code.

### Refactor code when done

We are proud of `qibuild` source code, and we want to have the best code quality as possible.

In order to do that, we never hesitate to:

- do massive refactoring (the Python API is not stable yet at all)
- change the config files format (they are not stable either yet)

An other good thing about TDD is that is also tells you when to just stop coding, thus preventing feature creep.

If you started by adding specifications in the documentation, you know what you are doing, you had no problem writing the test cases, and as soon as all the tests pass, you know you can stop coding.

## Links

- Chapter about unit testing from the “Dive Into Python book”
- What Killed Smalltalk Could Kill Ruby, too (A fascinating talk by Robert Martin at RailsConf 09, about programming in general, with a focus on unit testing at the end)
- Fast Test, Slow Test A talk by Gary Bernhardt at PyCon 2012

See also

- Running qiBuild test suite
- Writing new tests

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