Improve your software quality with Property-Based Testing

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@yot88 PROPERTY-BASED TESTING (PBT) PBT

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@yot88 A DEV STORY Scott Wlaschin https://fr.slideshare.net/ScottWlaschin/an-introduction-to-property-based-testing

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A dev story

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A dev story

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A dev story

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A dev story

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@yot88 Which tests would you have been written ?

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A dev story

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A dev story

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A dev story

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A dev story

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A dev story

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A dev story

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A dev story

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A dev story What is our code coverage ?

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No content

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@yot88 Let’s add new tests

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@yot88 A dev story WTF ?

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@yot88 A dev story

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A dev story

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@yot88 A dev story

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@yot88 A dev story Let’s rethink the approach Do not use specific examples anymore

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@yot88 A dev story We can’t test the add by using +

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@yot88 PBT to the rescue

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@yot88 Testing with requirements / properties What about parameter order ? (compared to subtract) The parameter order does not matter when_I_add_two_numbers_the_result_should_not_depend_on_parameter_order “add 1” twice is the same as doing “add 2” (compared to multiply) What about adding 0 ? Adding 0 does nothing We are confident the implementation is correct if we test these properties It applies to all inputs

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Specification with properties Associativity property Commutativity property Identity property

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@yot88 Is there a simpler way to do it ?

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@yot88 What is Property based testing ? A property is the combination of an invariant with an input values generator. For each generated value, the invariant is treated as a predicate and checked whether it yields true or false for that value. As soon as there is one value which yields false, the property is said to be falsified, and checking is aborted. If a property cannot be invalidated after a specific amount of sample data, the property is assumed to be satisfied.

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What is Property based testing PBT

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@yot88 What is Property based testing ? Describe the input Describe the properties of the output Have the computer try lots of random examples Check if it fails

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@yot88 What is Property based testing

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@yot88 What can we do in java ?

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Junit-quickcheck https://pholser.github.io/junit-quickcheck/site/0.9.1/

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Junit-quickcheck

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Junit-quickcheck

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Junit-quickcheck

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Junit-quickcheck

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Junit-quickcheck

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Junit-quickcheck

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Junit-quickcheck

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Junit-quickcheck

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Junit-quickcheck

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@yot88 And in real life ?

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Real life

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Real life – identify properties The balance should be decremented of the withdrawal amount when I have enough money Or the overdraft is authorized for my account I must not be allowed to withdraw when The withdraw amount is over my max withdrawal amount My balance is insufficient, and overdraft is not authorized for my account

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Real life – custom generators To use them Or use the serviceloader by creating a file called: com.pholser.junit.quickcheck.generator.Generator in META-INF/services

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Real life – implementation

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@yot88 Vavr stuff ?

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PBT in vavr

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PBT in vavr

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@yot88 And so ?

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@yot88 PBT and example-based tests PBTs are more general 1 property-based test can replace many example-based tests PBTs can reveal edge cases Nulls, negative numbers, weird strings (encoding for example), … PBTs ensures deep understanding of the business invariants Example-based tests are still helpful though Easier to understand & start with

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@yot88 What could we do together ? NEXT STEPS

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@yot88 Resources An introduction to property-based testing – Scott Wlaschin Property-Based Testing for everyone - Romeu Moura (Video) Property-based testing in Java with JUnit-Quickcheck - Kenny Baas-Schwegler https://baasie.com/2017/06/12/property-based-testing-in-java-with-junit-quickcheck-part-2- generators/ Property Testing with vavr junit-quickcheck documentation