Unit test for the 'none' type in Python

Question

How would I go about testing for a function that does not return anything?

For example, say I have this function:

def is_in(char):
    my_list = []
    my_list.append(char)

and then if I were to test it:

class TestIsIn(unittest.TestCase):

    def test_one(self):
    ''' Test if one character was added to the list'''
    self.assertEqual(self.is_in('a'), # And this is where I am lost)

I don't know what to assert the function is equal to, since there isn't any return value that I could compare it to.

Would assertIn work?

Solution 1

All Python functions return something. If you don't specify a return value, None is returned. So if your goal really is to make sure that something doesn't return a value, you can just say

self.assertIsNone(self.is_in('a'))

(However, this can't distinguish between a function without an explicit return value and one which does return None.)

Solution 2

The point of a unit test is to test something that the function does. If it's not returning a value, then what is it actually doing? In this case, it doesn't appear to be doing anything, since my_list is a local variable, but if your function actually looked something like this:

def is_in(char, my_list):
    my_list.append(char)

Then you would want to test if char is actually appended to the list. Your test would be:

def test_one(self):
    my_list = []
    is_in('a', my_list)
    self.assertEqual(my_list, ['a'])

Since the function does not return a value, there isn't any point testing for it (unless you need make sure that it doesn't return a value).