Python in a Nutshell, 2nd Edition

Testing

In this chapter, I distinguish between two rather different kinds of testing: unit testing and system testing. Testing is a rich and important field, and many more distinctions could be drawn, but I focus on the issues of most immediate importance to software developers. Many developers are reluctant to spend time on testing, seeing it as time subtracted from “real” development, but this attitude is short-sighted: defects are easier to fix the earlier you find out about them, so each hour spent developing tests can amply pay back for itself by finding defects ASAP, thus saving many hours of debugging that would otherwise have been needed in later phases of the software development cycle.

Unit Testing and System Testing

Unit testing means writing and running tests to exercise a single module or an even smaller unit, such as a class or function. System testing (also known as functional or integration testing) involves running an entire program with known inputs. Some classic books on testing also draw the distinction between white-box testing, done with knowledge of a program’s internals, and black-box testing, done without such knowledge. This classic viewpoint parallels, but does not exactly duplicate, the modern one of unit versus system testing.

Unit and system testing serve different goals. Unit testing proceeds apace with development; you can and should test each unit as you’re developing it. One modern approach is known as test-driven development (TDD): for each feature ...