Vaidas Pilkauskas and Tadas Ščerbinskas - Can you trust your tests?
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The document discusses mutation testing as a way to evaluate test quality and coverage. Mutation testing works by modifying the code being tested in small ways (called mutants) and checking if tests detect the changes. This helps identify code that is not fully covered by tests. While it can improve test quality, mutation testing is computationally expensive and challenges include equivalent mutants and infinite loops. The document provides an overview of how mutation testing works and some tools available to implement it.
![array = [a, b, c]
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![//test
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max(a) {
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}](https://image.slidesharecdn.com/canyoutrustyourtestsagiletourkaunas-151024163831-lva1-app6892/85/Vaidas-Pilkauskas-and-Tadas-Scerbinskas-Can-you-trust-your-tests-27-320.jpg)
![//test
max([0]) == 0
max([1]) == 1
//implementation
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![//test
max([0]) == 0
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max([0, 2] == 2
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![Mutation
// test
max([0]) == 0
max([1]) == 1
max([0, 2] == 2
// implementation
max(a) {
m = a.first
for (e in a)
if (e > m)
m = e
return m
}](https://image.slidesharecdn.com/canyoutrustyourtestsagiletourkaunas-151024163831-lva1-app6892/85/Vaidas-Pilkauskas-and-Tadas-Scerbinskas-Can-you-trust-your-tests-31-320.jpg)
![Mutation
// test
max([0]) == 0
max([1]) == 1
max([0, 2] == 2
// implementation
max(a) {
m = a.first
for (e in a)
if (true)
m = e
return m
}](https://image.slidesharecdn.com/canyoutrustyourtestsagiletourkaunas-151024163831-lva1-app6892/85/Vaidas-Pilkauskas-and-Tadas-Scerbinskas-Can-you-trust-your-tests-32-320.jpg)
![// test
max([0]) == 0
max([1]) == 1
max([0, 2]) == 2
// implementation
max(a) {
return a.last
}
Baby steps matter](https://image.slidesharecdn.com/canyoutrustyourtestsagiletourkaunas-151024163831-lva1-app6892/85/Vaidas-Pilkauskas-and-Tadas-Scerbinskas-Can-you-trust-your-tests-33-320.jpg)
![// test
max([0]) == 0
max([1]) == 1
max([0, 2]) == 2
max([2, 1]) == 2
// implementation
max(a) {
m = a.first
for (e in a)
if (e > m)
m = e
}](https://image.slidesharecdn.com/canyoutrustyourtestsagiletourkaunas-151024163831-lva1-app6892/85/Vaidas-Pilkauskas-and-Tadas-Scerbinskas-Can-you-trust-your-tests-34-320.jpg)
Vaidas Pilkauskas and Tadas Ščerbinskas - Can you trust your tests?
- 1. Can You Trust Your Tests? 2015 Vaidas Pilkauskas & Tadas Ščerbinskas
- 2. Van Halen
- 4. Van Halen’s 1982 Tour Rider
- 6. Agenda 1. Test quality & code coverage 2. Mutation testing in theory 3. Mutation testing in practice
- 7. Prod vs. Test code quality Code has bugs. Tests are code. Tests have bugs.
- 10. “Program testing can be used to show the presence of bugs, but never to show their absence!” - Edsger W. Dijkstra
- 11. Code Coverage
- 12. Types of Code Coverage Lines Branches Instructions Cyclomatic Complexity Methods & more
- 15. Lines boolean doFoo() { return "a" + "b"; } assertThat(a.doFoo(), is("ab"));
- 16. Lines boolean doFoo(boolean arg) { return arg ? "a" : "b"; } assertThat(a.doFoo(true), is("a"));
- 17. SUCCESS: 26/26 (100%) Tests passed
- 18. Branches boolean doFoo(boolean arg) { return arg ? "a" : "b"; } assertThat(a.doFoo(true), is("a")); assertThat(a.doFoo(false), is("b"));
- 19. Can you trust 100% coverage? Code coverage can only show what is not tested. For interpreted languages 100% code coverage is kind of like full compilation.
- 20. Code Coverage can be gamed On purpose or by accident
- 21. Mutation testing
- 22. Mutation testing Changes your program code and expects your tests to fail.
- 23. Wait! What exactly is a mutation? def isFoo(a) { return a == foo; } def isFoo(a) { return a != foo; } def isFoo(a) { return true; } def isFoo(a) { return null; } > > >
- 24. Terminology Applying a mutation to some code creates a mutant. If test passes - mutant has survived. If test fails - mutant is killed.
- 25. Failing is the new passing
- 26. array = [a, b, c] max(array) == ???
- 27. //test max([0]) == 0 //implementation max(a) { return 0 }
- 28. //test max([0]) == 0 max([1]) == 1 //implementation max(a) { return a.first }
- 29. //test max([0]) == 0 max([1]) == 1 max([0, 2] == 2 //implementation max(a) { m = a.first for (e in a) if (e > m) m = e return m }
- 30. Coverage
- 31. Mutation // test max([0]) == 0 max([1]) == 1 max([0, 2] == 2 // implementation max(a) { m = a.first for (e in a) if (e > m) m = e return m }
- 32. Mutation // test max([0]) == 0 max([1]) == 1 max([0, 2] == 2 // implementation max(a) { m = a.first for (e in a) if (true) m = e return m }
- 33. // test max([0]) == 0 max([1]) == 1 max([0, 2]) == 2 // implementation max(a) { return a.last } Baby steps matter
- 34. // test max([0]) == 0 max([1]) == 1 max([0, 2]) == 2 max([2, 1]) == 2 // implementation max(a) { m = a.first for (e in a) if (e > m) m = e }
- 35. Tests’ effectiveness is measured by number of killed mutants by your test suite.
- 36. It’s like hiring a white-hat hacker to try to break into your server and making sure you detect it.
- 37. What if mutant survives ● Simplify your code ● Add additional tests ● TDD - minimal amount of code to pass the test
- 38. Challenges 1. High computation cost - slow 2. Equivalent mutants - false negatives 3. Infinite loops
- 39. Equivalent mutations // Original int i = 0; while (i != 10) { doSomething(); i += 1; } // Mutant int i = 0; while (i < 10) { doSomething(); i += 1; }
- 40. Infinite Runtime // Original while (expression) doSomething(); // Mutant while (true) doSomething();
- 41. Disadvantages ● Can slow down your TDD rhythm ● May be very noisy
- 42. Let’s say we have codebase with: ● 300 classes ● around 10 tests per class ● 1 test runs around 1ms ● total test suite runtime is about 3s Is it really slow? Let’s do 10 mutations per class ● We get 3000 (300 * 10) mutations ● runtime with all mutations is 150 minutes (3s * 3000)
- 43. Speeding it up Run only tests that cover the mutation ● 300 classes ● 10 tests per class ● 10 mutations per class ● 1ms test runtime ● total mutation runtime 10 * 10 * 1 * 300 = 30s
- 44. Speeding it up During development run tests that cover only your current changes
- 45. Usage scenarios ● Continuous integration ● TDD with mutation testing only on new changes ● Add mutation testing to your legacy project, but do not fail a build - produce warning report
- 46. Tools ● Ruby - Mutant ● Java - PIT ● And many tools for other languages
- 47. Summary ● Code coverage highlights code that is definitely not tested ● Mutation testing highlights code that definitely is tested ● Given non equivalent mutations, good test suite should work the same as a hash function
- 48. About us Vaidas Pilkauskas @liucijus ● Vilnius JUG co- founder ● Vilnius Scala leader ● Coderetreat facilitator ● Mountain bicycle rider ● Snowboarder Tadas Ščerbinskas @tadassce ● VilniusRB co- organizer ● RubyConfLT co- organizer ● RailsGirls Vilnius & Berlin coach ● Various board sports’ enthusiast
- 49. Credits A lot of presentation content is based on work by these guys ● Markus Schirp - author of Mutant ● Henry Coles - author of PIT ● Filip Van Laenen - working on a book
- 50. Q&A