Once you’ve flushed out the basic crashing bugs in your code, you’re going to want to look for more interesting things to test.
The next easiest thing to test is code where you know what the right answer is for every input.
Obviously in theory you think you know what the right answer is - you can just run the code. That’s not very helpful though, as that’s the answer you’re trying to verify.
But sometimes there is more than one way to get the right answer, and you choose the one you run in production not because it gives a different answer but because it gives the same answer faster.
- There might be a fancy but fast version of an algorithm and a simple but slow version of an algorithm.
- You might have a caching layer and be able to run the code with and without caching turned on, or with a different cache timeout.
- You might be moving to a new database backend to improve your scalability, but you still have the code for the old backend until you’ve completed your migration.
There are plenty of other ways this can crop up, but those are the ones that seem the most common.
Anyway, this creates an excellent use case for property based testing, because if two functions are supposed to always return the same answer, you can test that: Just call both functions with the same data and assert that their answer is the same.
Lets look at this in the fancy algorithm case. Suppose we implemented merge sort:
def merge_sort(ls): if len(ls) <= 1: return ls else: k = len(ls) // 2 return merge_sorted_lists( merge_sort(ls[:k]), merge_sort(ls[k:]) ) def merge_sorted_lists(x, y): result =  i = 0 j = 0 while i < len(x) and j < len(y): if x[i] <= y[j]: result.append(x[i]) i += 1 else: result.append(y[j]) j += 1 return result
We want a reference implementation to test it against, so lets also implement bubble sort:
def bubble_sort(ls): ls = list(ls) needs_sorting = True while needs_sorting: needs_sorting = False for i in range(1, len(ls)): if ls[i - 1] > ls[i]: needs_sorting = True ls[i - 1], ls[i] = ls[i], ls[i - 1] return ls
These should always give the same answer, so lets test that:
@given(lists(integers())) def test_bubble_sorting_is_same_as_merge_sorting(ls): assert bubble_sort(ls) == merge_sort(ls)
This gives us an error:
@given(lists(integers())) def test_bubble_sorting_is_same_as_merge_sorting(ls): > assert bubble_sort(ls) == merge_sort(ls) E assert [0, 0] ==  E Left contains more items, first extra item: 0 E Use -v to get the full diff foo.py:43: AssertionError ----- Hypothesis ----- Falsifying example: test_bubble_sorting_is_same_as_merge_sorting(ls=[0, 0])
What’s happened is that we messed up our implementation of merge_sorted_lists, because we forgot to include the elements left over in the other list once we’ve reached the end of one of them. As a result we ended up losing elements from the list, a problem that our simpler implementation lacks. We can fix this as follows and then the test passes:
def merge_sorted_lists(x, y): result =  i = 0 j = 0 while i < len(x) and j < len(y): if x[i] <= y[j]: result.append(x[i]) i += 1 else: result.append(y[j]) j += 1 result.extend(x[i:]) result.extend(y[j:]) return result
This technique combines especially well with Hypothesis’s stateful testing, because you can use it to then test different implementations of complex APIs. For example, Hypothesis uses this property together with stateful testing to verify that the different implementations of its example database behave identically.