Development with Python

CHAPTER 22

Fast Tests, Slow Tests, and Hot Lava

The database is Hot Lava!

—

Right up until

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 be‐

tween 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 sub‐

tleties 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 mud‐

dling 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 a single function, for example, written in such a

way that only that function can make it fail. If the function depends on another

system, and breaking that system breaks our test, we have an

integrated

test. That

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