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
   

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2. What to expect
   • Why automated testing?
   

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3. What to expect
   • Why automated testing?
   • Define property-based testing
   

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4. What to expect
   • Why automated testing?
   • Define property-based testing
   • Anatomy of a test
   

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5. What to expect
   • Why automated testing?
   • Define property-based testing
   • Anatomy of a test
   • Examples
   

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6. What to expect
   • Why automated testing?
   • Define property-based testing
   • Anatomy of a test
   • Examples
   • Next steps
   

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7. Why automated testing?
   

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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
   

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9. Questions?
   

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10. What is 

    property-based testing?
    

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11. Unit test
    • Specify inputs
    • Run code under test
    • Make assertions on output
    

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12. How many tests 

    are enough?
    

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13. Property-based test
    • Define a way to generate inputs
    • Run code under test
    • Make assertions about output
    • This is the tricky part!
    

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14. How can I assert anything
    about the output without
    knowing the input?
    

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15. A property is something
    your system exhibits for
    any input
    

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16. Sample properties
    • x + 0 = x
    • x * 1 = x
    • x + y = y + x
    • strlen(strpad(str, n)) === n
    

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17. Different paths, same destination
    http://fsharpforfunandprofit.com/posts/property-based-testing-2/
    

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18. There and back again
    http://fsharpforfunandprofit.com/posts/property-based-testing-2/
    

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19. Some things never change
    http://fsharpforfunandprofit.com/posts/property-based-testing-2/
    

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20. The more things change, the more they stay the same
    http://fsharpforfunandprofit.com/posts/property-based-testing-2/
    

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21. Hard to prove, easy to verify
    http://fsharpforfunandprofit.com/posts/property-based-testing-2/
    

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22. Two heads are better than one
    http://fsharpforfunandprofit.com/posts/property-based-testing-2/
    

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23. Danger!!
    Don’t reimplement your function in your test
    

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24. Questions?
    

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25. Eris
    PHPUnit extension for property-based testing
    

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26. Anatomy of a test
    

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27. Generator
    

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28. Filter
    

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29. Assertions
    

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30. Generator demo
    

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31. Questions?
    

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32. Examples
    

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33. DEMO
    

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34. Questions?
    

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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
    

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36. Questions?
    @jclermont
    

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37. Thank you!
    @jclermont

    

    

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