When you have a well thought-out external specification, it really gives you lots of good material from which to derive all the right tests while still treating

When you have a well thought-out external specification, it really gives you lots of good material from which to derive all the right tests while still treating your implementation as a black box. Unless you are implementing an established RFC or some other well-defined system, such a tight specification rarely exists in a software setting, and is very difficult to effectively design a priori. For this reason, techniques like test-driven development have arisen to try to make the programmer repeatedly and consistently assert their understanding of the specification via unit tests, while evolving those assumptions as needs and understandings change.