Show HN: PBT – A property-based testing library for Ruby

Show HN: PBT – A property-based testing library for Ruby(github.com)

196 points by ohbarye 2 years ago | 33 comments

Hello HN,

I introduce a property-based testing tool for Ruby. Ruby's ease of test-writing and rich ecosystem are widely acclaimed. However, property-based testing is not as widely used as in other languages such as Haskell or Elixir, and I think this is because there is no de facto testing tool.

This gem currently not only has the basic functionality of stateless property-based testing but also has the following features:

- Support verbose mode that allows you to see its shrinking procedure and algorithms.

- Support several concurrency/parallel executions of each test. As of now, Ractor/Thread/Process are available. (The default is sequential, considering benchmark results and actual use cases)

Happy hacking!

hdjrkaofbdjwm 2 years ago |

Great work, looks really clean! Check out Minithesis [1] and the paper explaining internal shrinking [2]. It will strongly improve your library's shrinking capabilities and prevent users from having to write their own shrinkers for custom objects in most cases.

[1] https://github.com/DRMacIver/minithesis

[2] https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.E...

ohbarye 2 years ago | |

Wow, I didn't know about them. Thanks for letting me know!

cortesoft 2 years ago |

Can this be used alongside rspec? Like have these be tests that are included in the rspec test run, and have the results alongside your other rspec tests?

ohbarye 2 years ago | |

The answer to both questions is yes.

This gem doesn't aim to reimplement a testing framework. It can also be considered just an assertion. You can use this alongside any testing framework like RSpec, minitest, and etc.

jack_riminton 2 years ago |

Interesting! I wonder how this could be applied to a Rails project (since that's what ruby is most used for). For example how would it know to test a wide selection of possible values for different variables if they're not explicitly set up?

parentheses 2 years ago | |

I imagine it wouldn't be too hard to build up your own data generators for Rails models (or other entities).

ohbarye 2 years ago | | |

Yeah, the easiest way is to generate attributes and instantiate any Rails model like the one below:

    Pbt.assert do
      Pbt.property(name: Pbt.printable_string, age: Pbt.integer(min: 0, max: 100)) do |name:, age:|
        user = User.new(name: name, age: age)
        # write your test here
      end
    end

Alifatisk 2 years ago | |

> Rails [...] that's what ruby is most used for

I wish that wasn't the case, I use Ruby heavily for gluing different systems together.

mikepurvis 2 years ago | | |

Outside of language level preferences, is there something about the ecosystem around Ruby that makes it particularly well suited to this? I would have thought Python was the glue language winner just because there are already bindings to the entire universe available for it.

parentheses 2 years ago |

Very specific question: when tests fail due to a single example, you suggest creating a specific assertion with the given seed.

I imagine the seed is used to generate data and depending on the order of your generators, it produces different results.

For example, in:

    PBT.assert(seed = ..) do
      PBT.property(PBT.integer, PBT.string) ...
    end

Would changing the order of parameters to `property` change the actual test case?

tybug 2 years ago | |

(not OP but I would be surprised if the answer wasn't) yes, because you're changing the order in which the random draws are interpreted. But this isn't a problem in practice because you generally aren't changing the generator in the middle of debugging a failure.

parentheses 2 years ago | | |

So, this means refactoring becomes potentially difficult. While the gem is still a great accomplishment and very useful, I'd have to engineer my way around this issue before using it with things like a Rails Model which could have changing shape.

@OP:

I wonder if the README (and possibly runner) should suggest writing a test-case that doesn't rely on PBT when the user wants to preserve a case for future testing.

The issue here is that if you're saving a singular example and it represents a weird corner case, it's totally conceivable that a small change will result in an invisible change to that test case.

Another idea: it'd be great if the test could simply take examples that are failing and add them to a `failing_examples.rb` or some such. I know I'd use a feature like this quite a bit.

ohbarye 2 years ago | | |

>yes, because you're changing the order in which the random draws are interpreted. But this isn't a problem in practice because you generally aren't changing the generator in the middle of debugging a failure.

Correct. The test inputs are determined by a seed and generators (including the order of generators).

bloopernova 2 years ago |

Can anyone recommend an introduction to PBT for someone who knows nothing about it?

Edited to add: D'oh! There's an explanation in the linked repo.

ohbarye 2 years ago | |

It's very basic content though, here's the slides of my presentation at RubyKaigi 2024. This also includes the PBT gems's explanations.

https://speakerdeck.com/ohbarye/unlocking-potential-of-prope...

Speaking of my personal story, I used the fast-check documentation and Fred Herbert's book (their links are in the README) to study.

arialdomartini 2 years ago | |

You might appreciate this little series of posts, specifically thought to provide a smooth introduction:

https://arialdomartini.github.io/property-testing

Lio 2 years ago |

Lovely stuff, thank you.

It would be cool if we could eventually make use of RBS or inline Sorbet or something better than both to get the types for property testing for free.

ohbarye 2 years ago | |

Good point. There's already such a project to run PBT based on RBS. I'm also keeping an eye on this.

https://github.com/ksss/raap

mberning 2 years ago |

Very cool. Wonder how much time this adds to a test suite as you incorporate it more. Might need to have some different test targets to keep things fast.

ohbarye 2 years ago | |

Thank you.

In my opinion, PBT should be used in combination with example-based testing. Besides, since example-based testing cases account for the majority of tests, I think it's rare that PBT's execution time is dominant.

As for the combination and usages, refs: https://medium.com/criteo-engineering/introduction-to-proper...

lijok 2 years ago |

Amazing work ! Happy to see any effort towards PBT.

ohbarye 2 years ago | |

Thank you. I hope the gem will make your testing life happier!

Lammy 2 years ago |

Excited to see libraries using Ractors as somebody who has also fallen in love with them ♡

ohbarye 2 years ago | |

I can really empathize. Hope the Ruby ecosystem gets more compatible with Ractor.

slackstation 2 years ago |

They missed the opportunity to name this library 'Double Shot'.

ohbarye 2 years ago | |

I love seeing people suggest clever names I couldn't have thought of.

Pbt.assert do Pbt.property(name: Pbt.printable_string, age: Pbt.integer(min: 0, max: 100)) do |name:, age:| user = User.new(name: name, age: age) # write your test here end end