In previous examples, we created test cases for the add_ten() function with various inputs. However, the actual logic of our tests really didn’t change. To decrease repetition, Python provides us a specific toolset for tests with only minor differences. This is known as test parameterization. By parameterizing tests, we can leverage the functionality of a single test to get a large amount of coverage of different inputs.

To accomplish test parameterization, the unittest framework provides us with the subTest context manager. Let’s refactor our previous test class to utilize it and see it in action:

import unittest # The function we want to test def times_ten(number): return number * 100 # Our test class class TestTimesTen(unittest.TestCase): # A test method def test_times_ten(self): for num in [0, 1000000, -10]: with self.subTest(): expected_result = num * 10 message = 'Expected times_ten(' + str(num) + ') to return ' + str(expected_result) self.assertEqual(times_ten(num), expected_result, message)

Here, in our test method test_times_ten(), instead of writing individual test cases for each input of 0, 10, and 1000000, we can test a collection of inputs by using a loop followed by a with statement and our subTest context manager.

By using subTest, each iteration of our loop is treated as an individual test. Python will run the code inside of the context manager on each iteration, and if one fails, it will return the failure as a separate test case failure.

Just like before, we are using the assertEqual() method to check the expected result, and we are expecting (due to an error in times_ten()) that the cases of using an input of -10 and 1000000 will fail.

Here is the new output:

FAIL: test_times_ten (__main__.TestTimesTen) (<subtest>)
Traceback (most recent call last):
  File "scratch.py", line 12, in test_times_ten
    self.assertEqual(times_ten(num), expected_result, message)
AssertionError: 100000000 != 10000000 : Expected times_ten(1000000) to return 10000000

FAIL: test_times_ten (__main__.TestTimesTen) (<subtest>)
Traceback (most recent call last):
  File "scratch.py", line 12, in test_times_ten
    self.assertEqual(times_ten(num), expected_result, message)
AssertionError: -1000 != -100 : Expected times_ten(-10) to return -100

Ran 1 test in 0.000s

FAILED (failures=2)

If we want to expand our test coverage, we can simply modify the list that our loop iterates over. We can test a range of thousands of inputs simply by using the context manager setup to achieve test parameterization.

Optionally, we can give our subtests better readability by making a small change in our code for the first argument of self.subTest(). The below code has most of our script omitted for brevity but uses the same script we executed above:

# ... more code above.. for num in [0, 1000000, -10]: with self.subTest(num): # ... more code below ....

This makes our test clearer, because our test error message goes from:

FAIL: test_times_ten (__main__.TestTimesTen) (<subtest>)


FAIL: test_times_ten (__main__.TestTimesTen) [1000000]

When working with large amounts of test inputs, it is much easier to distinguish which case failed. We can actually use any message we want as the first argument, but using the tested case is usually the best way to increase readability for ourselves and other developers.

By using test parameterization, we made our codebase much cleaner and more maintainable. Let’s get some practice by refactoring some of our previous tests!



Small World Air is growing and has added many more movie options to the entertainment system (we can see them inside of entertainment.py).

Let’s adjust our EntertainSystemTests class to make sure they all get tested. Replace the call to entertainment.get_daily_movie() with entertainment.get_daily_movies() (our new method).

Lastly, for better readability, update the variable name daily_movie to daily_movies. After updating this variable name, update the first argument in the call to self.assertIn() to use this new variable name.


Under our two variables, write a for loop that iterates over daily_movies and stores each iteration value into a variable called movie. For now, let’s simply print movie on each iteration.


Indent our self.assertIn() call to be inside the for loop and change the first argument in self.assertIn() from daily_movie to movie to represent the individual movies on each iteration of the loop.

Note: Creating this structure might be okay at first glance (and may even make you wonder why we need the context manager), but if we run our test, we will see that the test will fail in the middle of our movies collection and won’t check the rest (it stops at Black Widow and not Spiral)! This is because like many testing frameworks, unittest will fail and stop on the first failure it encounters.


Lastly, under our print statement of movie but before our assertIn() call, insert a self.subTest() to wrap our test method. To make sure we can distinguish test cases between each movie, pass a single argument of movie into self.subTest().

Don’t forget to preface the context manager with a with statement and indent our self.assertIn() statement. Now, we can observe testing multiple movies and if they are licensed or not.

Sign up to start coding

By signing up for Codecademy, you agree to Codecademy's Terms of Service & Privacy Policy.
Already have an account?