T H E T E S T S M E L L B E F O R E C H R I S T M A S 

T E S T S S M E L L B U T T H A T O F F E R S G O O D I N S I G H T S O N T H E C O D E Code smells Behaviour smells Project smells 

H A R D T O T E S T C O D E SYMPTOMS • some code is inherently difficult to test (GUI, threads, remote services) • tight coupling • private logic • lots of dependencies 

H A R D T O T E S T C O D E IMPACT • testing becomes very hard and time consuming • less tests will be written • does not scale 

H A R D T O T E S T C O D E POSSIBLE SOLUTIONS • learn to decouple code • separate logic from the async mechanism • keep test logic unconditional 

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O B S C U R E T E S T S SYMPTOMS • no communication of intent • hard to understand • hard to maintain • can hardly be used as docs 

O B S C U R E T E S T S CAUSES • too much information (eg: Eager test, Irrelevant Information) • too little information (eg: Mystery Guest) • lack of attention and care 

O B S C U R E T E S T S EXAMPLE: EAGER TEST • symptom: verifies too much functionality in a single test • cause: “mocks” a human doing a test, chaining lots of actions together • solution: create multiple, single-condition tests for better Defect Localisation 

O B S C U R E T E S T S MYSTERY GUEST • symptom: hard to see cause-effect relationship between input and expected outcome • cause: tests become harder to follow and more fragile • solution: setup inline, aided by helpers/builders 

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C O N D I T I O N A L T E S T L O G I C SYMPTOMS • presence of control structures in tests • some test code doesn’t get executed 

C O N D I T I O N A L T E S T L O G I C IMPACT • harder to write correct tests • maintenance overhead • decreased confidence in tests • increased chance of bugs in tests 

C O N D I T I O N A L T E S T L O G I C CAUSES • handling failure conditions • testing collections • testing conditional logic • optimising setup/teardown logic 

C O N D I T I O N A L T E S T L O G I C POSSIBLE SOLUTIONS • support substitutable deps • add guard assertions • define test-specific equality • enumerate inputs and outputs 

C O N D I T I O N A L T E S T L O G I C C O D E E X A M P L E 

H A P P Y T E S T I N G ! 

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