Test-Driven Development with Python

Chapter 22. Fast Tests, Slow Tests, and Hot Lava

The database is Hot Lava!

— Casey Kinsey

Right up until Chapter 19, almost all of the “unit” tests in the book should perhaps have been called integrated tests, because they either rely on the database, or they use the Django test client, which does too much magic with the middleware layers that sit between requests, responses, and view functions.

There is an argument that a true unit test should always be isolated, because it’s meant to test a single unit of software.

Some TDD veterans say you should strive to write “pure”, isolated unit tests wherever possible, instead of writing integrated tests. It’s one of the ongoing (occasionally heated) debates in the testing community.

Being merely a young whippersnapper myself, I’m only part way towards all the subtleties of the argument. But in this chapter, I’d like to try and talk about why people feel strongly about it, and try and give you some idea of when you can get away with muddling through with integrated tests (which I confess I do a lot of!), and when it’s worth striving for more “pure” unit tests.

Terminology: Different Types of Test

Isolated tests (“pure” unit tests) vs. integrated tests

The primary purpose of a unit test should be to verify the correctness of the logic of your application. An isolated test is one that tests exactly one chunk of code, and whose success or failure does not depend on any other external code. This is what I call a “pure” unit test: a test for ...