Comment by eru

You can think of property based tests as defining a vast 'universe' of tests. Like 'for all strings S and T, we should have to_upper(S) + to_upper(T) == to_upper(S+T)' or something like that. That defines an infinite set of individual test cases: each choice for S and T gives you a different test case.

Running all of these tests would take too long. So instead we take a finite sample from our universe, and only run these.

> With non-deterministic tests I would always wonder if it's going to fail randomly after the code is already in production.

You could always take the same sample, of course. But that means you only ever explore a very small fraction of that universe. So it's more likely you miss something. Remember: closing your eyes doesn't make the tiger go away.

If there are important cases you want to have checked every time, you can use the @example decorator in Hypothesis, or you can just write a traditional example based test.