UNIT TESTING BEYOND 

THE PYRAMID OF TESTS HAVE YOU SEEN THIS IMAGE BEFORE? ▸ Common knowledge? ▸ Some say that it means that you should write more unit tests than any other type ▸ Another interpretation is that unit tests are faster, and cheaper 

MOST UNIT TESTING IS WASTE? WHAT IF I TELL YOU 

ABOUT HOW WE UNIT TEST LET’S BE HONEST 

LET’S BE HONEST ABOUT HOW WE UNIT TEST WE ONLY TEST PARTIAL VALUES ▸ It would be impossible to test every possible output with every combination of the environment ▸ The combinatory of all those possibilities can easily go up to trillions of things to test 

LET’S BE HONEST ABOUT HOW WE UNIT TEST WE THINK COVERING ALL LINES IS ENOUGH ▸ Covering all lines is not enough, because not all possible combinations are covered ▸ We like to trick the system by breaking functions to smaller pieces so we can cover all the lines ▸ This makes certain functions or classes more difficult to read 

LET’S BE HONEST ABOUT HOW WE UNIT TEST WE WOULD RATHER NOT THINK ▸ We find workarounds to make tests pass ▸ If a failing test does not match our understanding, we just change it so that it passes 

LET’S BE HONEST ABOUT HOW WE UNIT TEST TESTS ARE ALSO CODE ▸ Code needs to maintained ▸ Code contains bug ▸ Tests are code ▸ Your tests need maintenance and also have bugs ▸ Many changes in code need coordinated changes in tests ▸ In many code bases there are more lines of tests than application code 

BE HUMBLE ABOUT WHAT YOUR UNIT TESTS CAN ACHIEVE James Coplien LET’S BE HONEST ABOUT HOW WE UNIT TEST 

WITH CONFIDENCE HOW CAN WE REDUCE WASTE 

REDUCING WASTE WITH CONFIDENCE FOCUS ON INFORMATION AND RISK ▸ Unit tests are meant to reduce the risk of deploying bad code in production ▸ You need information in order to reduce risk ▸ Tests that never fail are not contributing any information 

DELETE TESTS THAT HAVE NOT FAILED IN A YEAR, MAINTAINING THEM IS COSTLIER James Coplien REDUCING WASTE WITH CONFIDENCE 

TYPE A QUOTE HERE. DELETE TESTS THAT ONLY TEST THE OBVIOUS 

TYPE A QUOTE HERE. DELETE TESTS THAT DON’T BREAK AFTER CHANGING THE BEHAVIOUR OF THE CODE THEY TEST 

TYPE A QUOTE HERE. DELETE TESTS THAT FAIL AND YOU CANNOT UNDERSTAND THE BUSINESS VALUE THEY PROVIDE 

THE BULK OF UNITS TEST SHOULD BE FOR ALGORITHMS WHERE THERE IS AN INDEPENDENT ORACLE FOR SUCCESS James Coplien REDUCING WASTE WITH CONFIDENCE 

CONVERT UNIT TESTS TO ASSERTIONS 

HAVE MORE INTEGRATION AND SYSTEM TESTS 

WRITE THE DOCUMENTATION BEFORE YOU IMPLEMENT 

USE PHPSTAN AND PSALM MANY UNIT TESTS CAN BE REPLACED BY BETTER TYPING