It is a good idea to make tests as descriptive as possible – to achieve Tests as Documentation. Including the calculations in the test is a big part of this, and avoids the Hard Coded Test Data smell.

For example, if a test looks like this, it is hard to know what the problem is when it fails, as the test data is all hard coded. The test tells you very little about how the system under test (SUT) should behave, and so doesn’t act as documentation.

assert sut.wake_erosion_rate(0.03) == 0.8

However, if you remove this code smell, and include the calculation in the test, it becomes obvious what the problem is when there is a failure, and the test does now act as documentation.

You might also want to replace the 2.5 and 0.05 with the names of the constants they represent.

ambient_turbuluence = 0.03

    == 2.5 * ambient_turbuluence + 0.05

This article references the excellent XUnit Test Patterns book, which defines the most widely accepted lexicon on unit test patterns and practices.

Example Calculation

The rest of this article will discuss the example ConstructionMarginCalculator class, and will describe refactorings that simplify the tests and allow them to feasibly include the calculations.

The calculation is around 30 lines long, and is worth a quick glance now if you have the time. But if not, don’t worry, the rest of this post will still make sense.

There are a few if statements and a loop, and in total these lead to 2^9, or 512, paths through the code! Eek! It is clearly not feasible (or useful) to test all these, hence the need to find ways to make it easier.

This example code has an initial naive test, which is around 30 lines long. It doesn’t include the calculations, and introducing them would make the test even longer and more complicated, so would be hard to justify.

Each of the following sections describes a refactoring, and includes links to the refactored example code. These build on the existing test refactorings from XUnit Test Patterns.

The refactorings reduce the number of paths through the code and simplify the test. This means that fewer tests are needed, and that the tests become small and simple enough to include the calculations.

Extract Calculation From Loop

The code initially calculates values for a list of steps, which means that any test for it must take on this complication.

The setup of the test is harder, as we have to create a list as opposed to a single value. The assertions are harder as we have to assert on a list as opposed to a single value. Finally, we should have multiple tests (probably for 0, 1, and multiple items in the list).

The easy fix is simply to refactor the code so that it only calculates a single step. This displaces the iteration code to some other place, which might want testing. However, this can be tested using a Mock, so only the iteration needs to be tested (as opposed to the iteration and the calculation), which is simple, and potentially so simple that you don’t need to test it.

This refactoring means that instead of requiring 3 tests (for 0, 1, and multiple items in the list), there is now just one. This reduces the number of paths through the code from 2^9 to 2^7, or 128. This is already a lot better, but still too many to test!

Introduce Mockable Abstractions

The details of the inflation calculation aren’t shown in the example, but they are reasonably complicated.

To avoid this we can change the code to accept an InflationCalculator. The inflation calculation always uses the same date_of_financial_close, inflation_rate and inflation_mode, which means that it can take these in the constructor. This in turn means that the main calculation no longer requires the inflation_rate and inflation_mode.

Then in the tests we can create a mock InflationCalculator. This could for example always return a value of 2, which makes the overall calculation a lot simpler.

It is also easy to imagine that inflation calculations will happen in other parts of the code, so the abstraction will be widely useful.

This step means that instead of 4 conditional branches in the inflation calculation, there is now just one. This reduces the number of paths through the code from 2^7 to 2^3, or just 8. It is also necessary to test the InflationCalculator, and this has 4 branches, so needs 4 tests, but this still only makes 12 tests in total. Yay for loosely coupled code!

We now have a feasible number of tests to write, but including the calculation in the test is still going to be very cumbersome. Luckily we still have some refactorings left up our sleeve.

Test Conditional Branches in Isolation

The code branches based on certain conditionals. We can simply make some of these conditionals False, and then test each branch of the code in isolation. This way, each test only has to include the calculation for the branch that it is concerned with.

For example, we can set in_selling_mode to be False and step.start_of_step to be different to date_of_financial_close. This makes the test simple enough that it is feasible for it to perform the same calculation that the code does.

This in turn means that the test clearly communicates that the turbine_cost_including_margin should be the turbine_costs * fraction_of_spend * inflation. This helps readers understand the calculator, and achieves the goal of Tests as Documentation

At the moment the test is still quite long. However, because we are only testing a small part of the calculation, a lot of this information is now irrelevant (the any_double variable). This means we can now create a Test Data Builder, or use helper functions to make things more concise. We will see an example of this in the next refactoring.

Test Values in Isolation

“Test Conditional Branches in Isolation” is a useful technique, but it can still leave us with some complications if a value is calculated / updated in multiple branches.

A good example of this is balance_of_plant_cost_including_margin, which is set initially, and then updated in the if (in_selling_mode) branch.

Testing balance_of_plant_cost_including_margin in isolation allows the test to concentrate just on this one value / calculation, which means that a lot less setup is required. The Test Data Builder pattern hides Irrelevant Information, and the test becomes more concise and expressive.

Including the calculation continues to make the test clearly communicate its intent and act as documentation. Interestingly the test calculation code is no longer an exact copy of the SUT calculation code, as it already knows that it is in_selling_mode, so doesn’t need a conditional statement. This means that the test is simpler than the code, which helps avoid the Obscure Test smell.

Test Partial Values in Isolation

Sometimes the calculation of individual values is very complex, and can’t be split up by conditional branches. This can make it challenging to include the calculation in the test. construction_profit is a reasonable example of this, which is calculated as follows:

step.turbine_cost_including_margin = 
 turbine_costs * inflation * fraction_of_spend;

step.balance_of_plant_cost_including_margin = 
 balance_of_plant_costs_at_financial_close * inflation * fraction_of_spend;

step.construction_profit = 
 -1 * 
 (step.turbine_cost_including_margin + step.balance_of_plant_cost_including_margin) *

inflation, fraction_of_spend and epc_margin have a multiplicative effect, so if we set them to 1, they won’t have any effect and we can easily write a test for the rest of the logic.

step.turbine_cost_including_margin and step.balance_of_plant_cost_including_margin have an additive effect, so if we set them to 0, again they won’t have any effect and we can easily write a test for the rest of the logic.

Testing just a portion of construction_profit in isolation allows the test to concentrate just on this part of the calculation, which, as before, makes the test shorter and simpler, and avoids the Obscure Test smell.

Introduce the Blackboard Pattern

The Blackboard pattern is a more involved technique, and is often badly understood. But essentially it involves using a “blackboard” to break the dependencies within complicated calculations.

In this example the construction_profit depends on turbine_cost_including_margin and balance_of_plant_cost_including_margin, which are themselves calculated from the inputs. This makes testing harder.

In order to test the construction_profit you essentially also have to test turbine_cost_including_margin and balance_of_plant_cost_including_margin.

When we introduce the blackboard pattern, one calculator writes the turbine_cost_including_margin and balance_of_plant_cost_including_margin to the blackboard, and when calculating the construction_profit we read these values from the blackboard.

This breaks the connection between the two things, so when testing we can just add values for turbine_cost_including_margin and balance_of_plant_cost_including_margin to the blackboard.


When testing calculations, it is important to include the calculation in the tests. This avoids the Hard Coded Test Data smell, allows the tests to clearly express their intent and achieve Tests as Documentation.

The refactorings described in this article allow you to do this while also making sure that the tests are concise and understandable.