The dispute led to the formation of twoschools of unit testing: the classical (Detroit) school, and the London (mockist)school

Writing a new test requires more effort than examining anexisting one, mostly because you don’t write tests in a vacuum: you have to take intoaccount the underlying code. And so although I focus on unit tests, I also devote a sig-nificant portion of this book to discussing code design.

This is not to say that coverage metrics should take into account code paths inexternal libraries (they shouldn’t), but rather to show you that you can’t rely onthose metrics to see how good or bad your unit tests are.

igure 1.4 The branch metric is calculated as the ratio of thenumber of code branches exercised by the test suite and thetotal number of branches in the production code base

In software, entropy manifests in the form of code that tends to deteriorate. Eachtime you change something in a code base, the amount of disorder in it, or entropy,increases. If left without proper care, such as constant cleaning and refactoring, thesystem becomes increasingly complex and disorganized.