Hypothesis: Property-based testing for Python

We all know we should be writing tests, but coming up with tests for every edge case is hard work and you will inevitably miss some. In this talk, I'll show you how to use Hypothesis in your projects to find more bugs than you would imagine.

Transcript

1. Hypothesis Property-based testing for Python Rae Knowler PyCon Thailand 2019
2. Who I am @RaeKnowler Python (Django, CKAN) PHP, Go, JavaScript they/them/their https://www.flickr.com/photos/zurichtourism/5160475075
3. What is property-based testing?
4. @RaeKnowler #pyconth What is property-based testing? • Define properties rather than specific scenarios
5. @RaeKnowler #pyconth What is property-based testing? • Define properties rather than specific scenarios • Give the test input and check that properties hold
6. @RaeKnowler #pyconth What is property-based testing? • Define properties rather than specific scenarios • Give the test input and check that properties hold • Can automatically generate random inputs
7. What is property-based testing?
8. What is Hypothesis?
9. @RaeKnowler #pyconth What is Hypothesis? • A property-based testing library for Python • Inspired by QuickCheck (Haskell) • https://hypothesis.works/ • https://hypothesis.readthedocs.io
10. David MacIver and Sinister Katze @DRMacIver @sinister_katze
11. @RaeKnowler #pyconth Hypothesis • Generates randomised test input
12. @RaeKnowler #pyconth Hypothesis • Generates randomised test input • Shrinks examples to find the simplest failing input
13. @RaeKnowler #pyconth Hypothesis • Generates randomised test input • Shrinks examples to find the simplest failing input • Remembers interesting inputs
14. @RaeKnowler #pyconth Hypothesis • Generates randomised test input • Shrinks examples to find the simplest failing input • Remembers interesting inputs • Can deal with complex data types
15. @RaeKnowler #pyconth Hypothesis • Generates randomised test input • Shrinks examples to find the simplest failing input • Remembers interesting inputs • Can deal with complex data types • Works with pytest and unittest
16. Getting started with Hypothesis
17. @RaeKnowler #pyconth pip install hypothesis
18. @RaeKnowler #pyconth A very simple example
19. @RaeKnowler #pyconth A very simple example
20. @RaeKnowler #pyconth A very simple example
21. Slightly more advanced: Django integration
22. @RaeKnowler #pyconth pip install hypothesis[django]
23. @RaeKnowler #pyconth Django integration Screenshot of models here That might be too big so just the first one maybe?
24. @RaeKnowler #pyconth Django integration
25. @RaeKnowler #pyconth Django integration
26. @RaeKnowler #pyconth Django integration
27. @RaeKnowler #pyconth Django integration
28. @RaeKnowler #pyconth Django integration
29. Additional features
30. @RaeKnowler #pyconth Django integration • Hypothesis extra • Generate strategies for your Django models and forms • Examples obey validators • Generate child models ◦ e.g. a Company has Shops
31. @RaeKnowler #pyconth The Python scientific stack • Numpy strategies ◦ arrays() • Pandas strategies ◦ pandas.Index, pandas.Series, pandas.DataFrame • Both of these are extras
32. @RaeKnowler #pyconth Other extras • dpcontracts • lark • pytz • dateutil
33. @RaeKnowler #pyconth Rule-based stateful testing • Give Hypothesis a set of operations • It generates sequences of operations, looking for ones that cause failures
34. Summary
35. @RaeKnowler #pyconth Summary Use Hypothesis!
36. @RaeKnowler #pyconth Summary Example repo: https://github.com/bellisk/hypothesis-django-example Slides:
37. Thank you!