Test faster, fix more

This is one of the most common first questions about Hypothesis.

People generally assume that the number of tests run will depend on the specific strategies used, but that’s generally not the case. Instead Hypothesis has a fairly fixed set of heuristics to determine how many times to run, which are mostly independent of the data being generated.

But how many runs is that?

The short answer is 200. Assuming you have a default configuration and everything is running smoothly, Hypothesis will run your test 200 times.

The longer answer is “It’s complicated”. It will depend on the exact behaviour of your tests and the value of some settings. In this article I’ll try to clear up some of the specifics of which settings affect the answer and how.

Advance warning: This is a set of heuristics built up over time. It’s probably not the best choice of heuristics, but it mostly seems to work well in practice. It will hopefully be replaced with a simpler set of rules at some point.

The first setting that affects how many times the test function will be called is the timeout setting. This specifies a maximum amount of time for Hypothesis to run your tests for. Once that has exceeded it will stop and not run any more (note: This is a soft limit, so it won’t interrupt a test midway through).

The result of this is that slow tests may get run fewer times. By default the timeout is one minute, which is high enough that most tests shouldn’t hit it, if your tests take somewhere in the region of 300-400ms on average they will start to hit the timeout.

The timeout is respected regardless of whether the test passes or fails, but other than that the behaviour for a passing test is very different from a failing one.

Passing tests

For the passing case there are two other settings that affect the answer: max_examples and max_iterations.

In the normal case, max_examples is what you can think of as the number of test runs. The difference comes when you start using assume or filter (and a few other cases).

Hypothesis distinguishes between a valid test run and an invalid one

  • if assume has been called with a falsey value or at some point in the generation process it got stuck (e.g. because filter couldn’t find any satisfying examples) it aborts the example and starts again from the beginning. max_examples counts only valid examples while max_iterations counts all examples, valid or otherwise. Some duplicate tests will also be considered invalid (though Hypothesis can’t distinguish all duplicates. e.g. if you did integers().map(lambda x: 1) it would think you had many distinct values when you only had one). The default value for max_iterations is currently 1000.

To see why it’s important to have the max_iterations limit, consider something like:

from hypothesis import given, assume, strategies as st

def test_stuff(i):

Then without a limit on invalid examples this would run forever.

Conversely however, treating valid examples specially is useful because otherwise even casual use of assume would reduce the number of tests you run, reducing the quality of your testing.

Another thing to note here is that the test with assume(False) will actually fail, raising:

hypothesis.errors.Unsatisfiable: Unable to satisfy assumptions of hypothesis test_stuff. Only 0 examples considered satisfied assumptions

This is because of the min_satisfying_examples setting: If Hypothesis couldn’t find enough valid test cases then it will fail the test rather than silently doing the wrong thing.

min_satisfying_examples will never increase the number of tests run, only fail the test if that number of valid examples haven’t been run. If you’re hitting this failure you can either turn it down or turn the timeout or max_iterations up. Better, you can figure out why you’re hitting that and fix it, because it’s probably a sign you’re not getting much benefit out of Hypothesis.

Failing tests

If in the course of normal execution Hypothesis finds an example which causes your test to fail, it switches into shrinking mode.

Shrinking mode tries to take your example and produce a smaller one. It ignores max_examples and max_iterations but respects timeout. It also respects one additional setting: max_shrinks.

max_shrinks is the maximum number of failing tests that Hypothesis will see before it stops. It may try any number of valid or invalid examples in the course of shrinking. This is because failing examples tend to be a lot rarer than passing or invalid examples, so it makes more sense to limit based on that if we want to get good examples out at the end.

Once Hypothesis has finished shrinking it will run your test once more to replay it for display: In the final run only it will print out the example and any notes, and will let the exception bubble up to the test runner to be handled as normal.

In Conclusion

“It’s complicated”.

These heuristics are probably not the best. They’ve evolved over time, and are definitely not the ones that you or I would obviously come up with if you sat down and designed the system from scratch.

Fortunately, you’re not expected to know these heuristics by heart and mostly shouldn’t have to. I’m working on a new feature that will help show how many examples Hypothesis has tried and help debug why it’s stopped at that point. Hopefully it will be coming in a release in the near future.