Assertion statements are a good start to ensuring our programs are being tested, but they don’t necessarily tell us what we should test. Generally, we can start by testing the smallest unit of a program.

For example, in the real world, if we were testing the functionality of a door, we could test a multitude of units. The handle could be an example of a single unit that we must check to make sure a door functions, followed by the hinges and maybe even the lock.

In programming, these types of individual tests are called unit tests. Like our door handle, we can test a single unit of a program, such as a function, loop, or variable. A unit test validates a single behavior and will make sure all of the units of a program are functioning properly.

Let’s say we wanted to test a single function (a single unit). To test a single function, we might create several test cases. A test case validates that a specific set of inputs produces an expected output for the unit we are trying to test. Let’s examine a test case for our times_ten() function from the previous exercise:

# The unit we want to test def times_ten(number): return number * 100 # A unit test function with a single test case def test_multiply_ten_by_zero(): assert times_ten(0) == 0, 'Expected times_ten(0) to return 0'

Great, now we have a simple test case that validates that times_ten() is behaving as expected for a valid input of 0! We can improve our testing coverage of this function by adding some more test cases with different inputs. A common approach is to create test cases for specific edge case inputs as well as reasonable ones. Here is an example of testing two extreme inputs:

def test_multiply_ten_by_one_million(): assert times_ten(1000000) == 10000000, 'Expected times_ten(1000000) to return 10000000 def test_multiply_ten_by_negative_number(): assert times_ten(-10) == -100, 'Expected times_ten(-10) to return -100'

Now we have several test cases for a wide variety of inputs: a large number, a negative number, and zero. We can create as many test cases as we see fit for a single unit, and we should try to test all the unique types of inputs our unit will work with.

Now, let’s create a variety of unit tests for another feature of Small World Air.



At Small Air World, every plane seat has a monitor which displays the nearest emergency exit.

This monitor relies on a function called get_nearest_exit(), which takes a row number and then returns an exit location depending on where the row is. Let’s make sure our function is working properly by creating a unit test.

Create a function called test_row_1() that will host a test case. Inside the function, assert that a call of get_nearest_exit(1) should equal to 'front', along with a message, 'The nearest exit to row 1 is in the front!'.


Create another test case function called test_row_20().

Inside the function, call get_nearest_exit(20) and assert that the return value is equal to 'middle', along with the message, 'The nearest exit to row 20 is in the middle!'


Finally, create another test case function called test_row_40().

Inside the function, call get_nearest_exit(40) and assert that the return value is equal to 'back', along with the message, 'The nearest exit to row 40 is in the back!'


At the bottom of the file, call each of the three test functions we created. What would be the expected output?


Looks like our tests caught a logic error in our function get_nearest_exit()! If the row number is larger than 30, we actually want to return 'back'.

Adjust the function and fix the error so all of our tests pass (we should see no output).

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