Property-based testing: work harder, not smarter

Testing is an important part of software development, but how many tests is enough? Should you write a test for every possible value that could be submitted to a function? Property-based testing is a technique that can help you with this conundrum. Instead of writing a specific set of tests, write some generalized tests and let a tool randomly test inputs against your code.

I'll explain in more detail what property-based testing is, how it works, and show some practical examples using Eris, a new PHPUnit extension modeled after Haskell's QuickCheck library.

Transcript

1. Property-based Testing
 Work smarter, not harder Joel Clermont
 @jclermont
2. What to expect • Why automated testing?
3. What to expect • Why automated testing? • Define property-based testing
4. What to expect • Why automated testing? • Define property-based testing • Anatomy of a test
5. What to expect • Why automated testing? • Define property-based testing • Anatomy of a test • Examples
6. What to expect • Why automated testing? • Define property-based testing • Anatomy of a test • Examples • Next steps
7. Why automated testing?
8. Automated testing • We already test manually • Developers like to save time • Make the computer work for you • Bonus: Code design improves with unit testing
9. Questions?
10. What is 
 property-based testing?
11. Unit test • Specify inputs • Run code under test • Make assertions on output
12. How many tests 
 are enough?
13. Property-based test • Define a way to generate inputs • Run code under test • Make assertions about output • This is the tricky part!
14. How can I assert anything about the output without knowing the input?
15. A property is something your system exhibits for any input
16. Sample properties • x + 0 = x • x * 1 = x • x + y = y + x • strlen(strpad(str, n)) === n
17. Different paths, same destination http://fsharpforfunandprofit.com/posts/property-based-testing-2/
18. There and back again http://fsharpforfunandprofit.com/posts/property-based-testing-2/
19. Some things never change http://fsharpforfunandprofit.com/posts/property-based-testing-2/
20. The more things change, the more they stay the same http://fsharpforfunandprofit.com/posts/property-based-testing-2/
21. Hard to prove, easy to verify http://fsharpforfunandprofit.com/posts/property-based-testing-2/
22. Two heads are better than one http://fsharpforfunandprofit.com/posts/property-based-testing-2/
23. Danger!! Don’t reimplement your function in your test
24. Questions?
25. Eris PHPUnit extension for property-based testing
26. Anatomy of a test
27. Generator
28. Filter
29. Assertions
30. Generator demo
31. Questions?
32. Examples
33. DEMO
34. Questions?
35. Next steps • http://fsharpforfunandprofit.com/posts/property- based-testing/ • http://fsharpforfunandprofit.com/posts/property- based-testing-2/ • midwest.io - Jessica Kerr 
 https://www.youtube.com/watch?v=shngiiBfD80 • https://github.com/giorgiosironi/eris
36. Questions? @jclermont
37. Thank you! @jclermont