For a better reading experience, check out . this article on my website From time to time I like to read documentation of modules I know well. The python documentation is not a pleasant read but sometimes you strike a gem. I think Same thing but slightly different Distinguishing Test Iterations Let’s start with a simple function to check if a number is even def is_even(n):return n % 2 == 0 And a simple test class TestIsEven(TestCase): def test\_should\_be\_even(self): self.assertTrue(is\_even(2)) Nice, let’s add some more cases: class TestIsEven(TestCase):... def test\_zero\_should\_be\_even(self): self.assertTrue(is\_even(0)) def test\_negative\_should\_be\_even(self): self.assertTrue(is\_even(-2)) This is a simple example and we copied code three times. Let’s try to do better by writing a loop to iterate values we expect to be even: class TestIsEven(TestCase): def test\_should\_all\_be\_even(self): for n in (2, 0, -2, 11): self.assertTrue(is\_even(n)) This is starting to look more elegant, so I added an odd value, 11, to fail the test. Let’s run the test and see what it looks like: what is the problem? F===================================================FAIL: test_should_all_be_even (__main__.TestIsEven)— — —— — — — — — — — — — — — — — — — — — — — — — — Traceback (most recent call last): File “subtest.py”, line 18, in test_should_all_be_evenself.assertTrue(is_even(n))AssertionError: False is not true It failed as expected, but ? which value failed Enter subTest In python 3.4 there is a new feature called . Lets see it in action: subTest class TestIsEven(TestCase): def test\_should\_all\_be\_even(self): for n in (0, 4, -2, 11): **with self.subTest(n=n):** self.assertTrue(is\_even(n)) Running this test produces the following output: F==========================================================FAIL: test_should_all_be_even (__main__.TestIsEven) ( )— — — — — — — — — — — — — — — — — — — — — — — — — — — — — n=11 Traceback (most recent call last): File “subtest.py”, line 23, in test_should_all_be_evenself.assertTrue(is_even(n))AssertionError: False is not true So which value failed? 11! I . t’s in the title How multiple failures look like? F===========================================================FAIL: test_should_all_be_even (__main__.TestIsEven) ( )— — — — — — — — — — — — — — — — — — — — — — — — — — — — — n=3 Traceback (most recent call last): File “subtest.py”, line 23, in test_should_all_be_evenself.assertTrue(is_even(n))AssertionError: False is not true ==========================================================FAIL: test_should_all_be_even (__main__.TestIsEven) ( )— — — — — — — — — — — — — — — — — — — — — — — — — — — — — n=5 Traceback (most recent call last): File “subtest.py”, line 23, in test_should_all_be_evenself.assertTrue(is_even(n))AssertionError: False is not true ==========================================================FAIL: test_should_all_be_even (__main__.TestIsEven) ( )— — — — — — — — — — — — — — — — — — — — — — — — — — — — — n=11 Traceback (most recent call last): File “subtest.py”, line 23, in test_should_all_be_evenself.assertTrue(is_even(n))AssertionError: False is not true Exactly as if we wrote three separate test cases. Profit!
