[Tutor] unit testing - Separate methods or group tests together?

[Steven D'Aprano]

Modulok wrote:
> List,
> 
> When you create unit tests, do you group tests for a given function
> together into one unit test method, or do you prefer to have separate
> test methods for every assert statement?

Each test method should test one thing. That doesn't necessarily mean 
one assert, but one conceptual thing. Here's an example from one of my 
test suites:


class QuartileSkewnessTest(unittest.TestCase):
     def testFailure(self):
         # Test that function correctly raises an exception if the
         # arguments are out of order.
         self.assertRaises(ValueError, stats.quartile_skewness, 2, 3, 1)
         self.assertRaises(ValueError, stats.quartile_skewness, 9, 8, 7)

     def testNan(self):
         # Test that the degenerate case where all three arguments are
         # equal returns NAN.
         self.assertTrue(math.isnan(stats.quartile_skewness(1, 1, 1)))
         self.assertTrue(math.isnan(stats.quartile_skewness(5, 5, 5)))

     def testSkew(self):
         # Test skew calculations.
         self.assertEqual(stats.quartile_skewness(3, 5, 7), 0.0)
         self.assertEqual(stats.quartile_skewness(0, 1, 10), 0.8)
         self.assertEqual(stats.quartile_skewness(0, 9, 10), -0.8)


In this particular case, I happen to have one test class for this 
function, but that's not always the case. Each test class is for a set 
of related tests (e.g. "test everything about this function", or "test 
that this same property holds for all eight of these functions", or 
similar). Within the test class, each test method is for one test, not 
necessarily one assert.


-- 
Steven