For a better reading experience, check out [this article on my website](https://hakibenita.com/the-best-new-feature-in-unittest-you-didnt-know-you-need).

From time to time I like to read documentation of modules _I think_ I know well. The python documentation is not a pleasant read but sometimes you strike a gem.

![](https://hackernoon.com/hn-images/1*N7qjoIW1UumCTXAHs2bXRw.jpeg)

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 **what is the problem?** I added an odd value, 11, to fail the test. Let’s run the test and see what it looks like:

F  
\===================================================  
FAIL: test\_should\_all\_be\_even (\_\_main\_\_.TestIsEven)  
— — —— — — — — — — — — — — — — — — — — — — — — — — 

Traceback (most recent call last):

File “subtest.py”, line 18, in test\_should\_all\_be\_even  
self.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 [**subTest**](https://docs.python.org/3.5/library/unittest.html#unittest.TestCase.subTest). Lets see it in action:

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\_even  
self.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\_even  
self.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\_even  
self.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\_even  
self.assertTrue(is\_even(n))  
AssertionError: False is not true

Exactly as if we wrote three separate test cases.

**Profit!**

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