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I am new to hypothesis and I am looking for a way to generate a pair of similar recursive objects.

My strategy for a single object is similar to this example in the hypothesis documentation.

I want to test a function which takes a pair of recursive objects A and B and the side effect of this function should be that A==B.

My first approach would be to write a test which gets two independent objects, like:

@given(my_objects(), my_objects())
def test_is_equal(a, b):
    my_function(a, b)
    assert a == b

But the downside is that hypothesis does not know that there is a dependency between this two objects and so they can be completely different. That is a valid test and I want to test that too.

But I also want to test complex recursive objects which are only slightly different.

And maybe that hypothesis is able to shrink a pair of very different objects where the test fails to a pair of only slightly different objects where the test fails in the same way.

stefan
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1 Answers1

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This one is tricky - to be honest I'd start by writing the same test you already have above, and just turn up the max_examples setting a long way. Then I'd probably write some traditional unit tests, because getting specific data distributions out of Hypothesis is explicitly unsupported (i.e. we try to break everything that assumes a particular distribution, using some combination of heuristics and a bit of feedback).

How would I actually generate similar recursive structures though? I'd use a @composite strategy to build them at the same time, and for each element or subtree I'd draw a boolean and if True draw a different element or subtree to use in the second object. Note that this will give you a strategy for a tuple of two objects and you'll need to unpack it inside the test; that's unavoidable if you want them to be related.

Seriously do try just cracking up max_examples on the naive approach first though, running Hypothesis for ~an hour is amazingly effective and I would even expect it to shrink the output fairly well.

Zac Hatfield-Dodds
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