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I love property-based testing, especially the way it can uncover edge cases you wouldn't have thought about. Haven't used Hypothesis yet, but I once had FsCheck (property-based testing for F#) find a case where the data structure I was writing failed when there were exactly 24 items in the list and you tried to append a 25th. That was a test case I wouldn't have thought to write on my own, but the particular number (it was always the 25th item that failed) quickly led me to find the bug. Once my property tests were running overnight and not finding any failures after thousands and thousands of random cases, I started to feel a lot more confident that I'd nailed down the bugs.

I keep thinking I have a possible use case for property -based testing, and then I am up to my armpits in trying to understand the on-the-ground problem and don't feel like I have time to learn a DSL for describing all possible inputs and outputs when I already had an existing function (the subject-under-test) that I don't understand.

So rather than try to learn to black boxes at the same time , I fall back to "several more unit tests to document more edge cases to defensibly guard against"

Is there some simple way to describe this defensive programming iteration pattern in Hypothesis? Normally we just null-check and return early and have to deal with the early-return case. How do I quickly write property tests to check that my code handles the most obvious edge cases?

I love the idea of hypothesis! Haven't found a lot of use cases for it yet, I think the quick start example helps explain why.

Essentially, you're testing that "my_sort" returns the same as python's standard "sort". Of course, this means you need a second function that acts the same as the function you wrote. In real life, if you had that you probably wouldn't have written the function "my_sort" at all.

Obviously it's just a tiny example, but in general I often find writing a test that will cover every hypothetical case isn't realistic.

Anyone here use this testing in the wild? Where's it most useful? Do you have the issue I described? Is there an easy way to overcome it?

I've taught this in my testing courses. I find that (pytest) fixtures are often good enough for coming up with multiple tests but are simple enough to implement.

My first impression on property-based testing is that it's likely to introduce flakes that distract engineers from their task at hand, forcing them to investigate a new failed test in an area that they weren't even touching.

this is pretty sick - i imagine this speeds up hardening a lot. will try it out - thank you!

Is there something this nice for JS, with the decorators like that?

One cool application of this is schemathesis. I really enjoyed it, and I found more input validation bugs in my code than I can count. Very useful for public APIs.

Property based testing is fantastic.

Why is it not more popular?

My theory is that only code written in functional languages has complex properties you can actually test.

In imperative programs, you might have a few utils that are appropriate for property testing - things like to_title_case(str) - but the bulk of program logic can only be tested imperatively with extensive mocking.

Great project. I used it a lot, but now I mostly prefer ad hoc generators. Hypothesis combinators quickly become unmaintainable mess for non-trivial objects. Also, shrinking is not such a big deal when you can generate your data in a complexity-sorted order.

I'm using sqlglot to parse hundreds of old mysql back up files to find diffs of schemas. The joys of legacy code. I've found hypothesis to be super helpful for tightening up my parser. I've identified properties (invariants) and built some strategies. I can now generate many more permutations of DDL than I'd thought of before. And I have absolutely confidence in what I'm running.

I started off TDD covered the basics. Learned what I needed to learn about the files I'm dealing with, edge cases, sqlglot and then I moved onto Hypothesis for extra confidence.

I'm curious to see if it'll help with commands for APIs. I nothing else it'll help me appreciate how liberal my API is when perhaps I don't want it to be?

Make sure to read the docs and understand this well. It has its own vocabulary that can be very counterintuitive.

A little off topic but theirs’ this https://youtu.be/64t-gPC33cc this is a great video by jon about property testing in Rust

It’s been quite some time since I’ve been in the business of writing lots of unit tests, but back in the day, I found hypothesis to be a big force multiplier and it uncovered many subtle/embarrassing bugs for me. Recommend. Also easy and intuitive to use.

It seems to only implement a half of QuickCheck idea, because there is no counterexample shrinking. Good effort though! I wonder how hard would it be to derive generators for any custom types in python - probably not too hard, because types are just values

This approach has two fundamental problems.

1. It requires you to essentially re-implement the business logic of the SUT (subject-under-test) so that you can assert it. Is your function doing a+b? Then instead of asserting that f(1, 2) == 3 you need to do f(a, b) == a+b since the framework provides a and b. You can do a simpler version that's less efficient, but in the end of the day, you somehow need to derive the expected outputs from input arguments, just like your SUT does. Any logical error that might be slipped into your SUT implementation has a high risk of also slipping into your test and will therefore be hidden by the complexity, even though it would be obvious from just looking at a few well thought through examples.

2. Despite some anecdata in the comments here, the chances are slim that this approach will find edge cases that you couldn't think of. You basically just give up and leave edge case finding to chance. Testing for 0 or -1 or 1-more-than-list-length are obvious cases which both you the human test writer and some test framework can easily generate, and they are often actual edge cases. But what really constitutes an edge case depends on your implementation. You as the developer know the implementation and have a chance of coming up with the edge cases. You know the dark corners of your code. Random tests are just playing the lottery, replacing thinking hard.

Coincidentally, I recently stumbled upon a similar library for Go[1].

I haven't used it, or property-based testing, but I can see how it could be useful.

[1]: https://github.com/flyingmutant/rapid