Comment by dragonwriter

> However the grand-parent comment has a point that it's not easy to extract testable properties from code that's already written.

True, property-based testing does not solve the problem of deriving the intended behavior of code where behavior is not documented (either by requirements documents, or code comments, or tests that aren't just examples but clearly indicate the general behavior they are confirming, or...)

OTOH, PBT can be used to rapidly test hypotheses about the behavior (though intent is another question) of legacy code, which you are going to need to develop and validate to turn it into code that is maintainable (or even to replace it with something new, if you need to generally be compatible.) Determining whether deviations from a hypothesized behavior are intentional or bugs is still an exercise for the user, though whether the deviations are highly general or narrow to specific cases can help to illuminate that decision, and a library like Hypothesis will help determine that.