4.2 The importance of unit tests

Unit testing is one of the corner stones of Extreme Programming. Other XP concepts, such as collective ownership of code and continuous refinement, are only possible by having unit tests. Moreover, XP emphasizes that writing unit tests should be automated, that they should test everything in every class, and that they should run perfectly all the time.

I believe that these concepts apply directly to hardware design. In addition, unit tests are a way to manage simulation time. For example, a state machine that runs very slowly on infrequent events may be impossible to verify at the system level, even on the fastest simulator. On the other hand, it may be easy to verify it exhaustively in a unit test, even on the slowest simulator.

It is clear that unit tests have compelling advantages. On the other hand, if we need to test everything, we have to write lots of unit tests. So it should be easy and pleasant to create, manage and run them. Therefore, XP emphasizes the need for a unit test framework that supports these tasks. In this chapter, we will explore the use of the unittest module from the standard Python library for creating unit tests for hardware designs.

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