Development with Python

CHAPTER 19

Test Isolation, and “Listening to Your Tests”

In the last chapter, we made the decision to leave a unit test failing in the views layer

while we proceeded to write more tests and more code at the models layer to get it to

pass.

We got away with it because our app was simple, but I should stress that, in a more

complex application, this would be a dangerous decision. Proceeding to work on lower

levels while you’re not sure that the higher levels are

really

finished or not is a risky

strategy.

I’m grateful to Gary Bernhardt, who took a look at an early draft of

the previous chapter, and encouraged me to get into a longer discus‐

sion of test isolation.

Ensuring isolation between layers does involve more effort (and more of the dreaded

mocks!), but it can also help to drive out improved design, as we’ll see in this chapter.

Revisiting Our Decision Point: The Views Layer Depends on

Unwritten Models Code

Let’s revisit the point we were at half-way through the last chapter, when we couldn’t get

the

new_list

view to work because lists didn’t have the

.owner

attribute yet.

We’ll actually go back in time and check out the old codebase, so that we can see how

things would have worked if we’d used more isolated tests.

git checkout -b more-isolation

# a branch for this experiment

git reset --hard revisit_this_point_with_isolated_tests

Here’s what our failing tests looks like:

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