It is very common that the SUT depends on something else. The dependency could be anything like another function, object or external system and we may not be able to control their behavior all the time. A good unit test should not be affected by the changes in the dependencies of SUT.
So, a common technique is to replace the dependencies with fake versions that we can control.
For example, consider a function that takes age of the passenger and prints whether he/she is eligible for senior citizen discount when promotion is available.
The function which tells whether the promotion is available or not is defined in another module and we don’t have control over its behavior
module promotions
function isPromotionsAvailable():
.
.
.
module discounts
function isDiscountAvailable(age):
if promotions.isPromotionAvailable() && age > 60:
print("discount available")
else:
print("discount not available")
However, in order to test all the branches and conditions of isDiscountAvailable() function, we need to be able to control the result of isPromotionAvailable() function. So, we first create fake version of isPromotionAvailable() function in fakePromotions module with an option for us to control the result
module fakePromotions
offerPromotion = true
function isPromotionAvailable():
return offerPromotion
then replace the promotion with fakePromotion inside the test
function test_isDiscountAvailable():
// arrange
promotions = fakePromotions //replace promotions module with fakePromotions module
promotions.offerPromotion = true
age = 65
// act
result = discounts.isDiscountAvailable(age)
Depending on the language and framework we use, the way we replace the real dependency with fake one will be different.