While the previous example was simple to test, most code will be much more complex. It would be very tedious to have to perform these tests manually. Our time would be better spent writing automated tests.

Luckily, Python provides an easy way to perform simple tests in our code - the assert statement. An assert statement can be used to test that a condition is met. If the condition evaluates to False, an AssertionError is raised with an optional error message.

The general syntax looks like this:

assert <condition>, 'Message if condition is not met'

Consider the following example that demonstrates the assert statement paired with a function called times_ten. Note there is a bug in the function for demonstration purposes.

def times_ten(number): return number * 100 result = times_ten(20) assert result == 200, 'Expected times_ten(20) to return 200, instead got ' + str(result)

Here, we want to test if our times_ten() function works as intended. We use the assert statement to evaluate the expression result == 200 since we expect that our function would return 200 given an input of 20. Since this is not the case, this expression evaluates to False (there is a bug in times_ten - it actually multiplies by 100!), we get the following exception:

AssertionError: Expected times_ten(20) to return 200, instead got 2000

An assert statement is a quick and powerful way to verify that a program is in the correct state. They can be used to catch mistakes early and make sure we avoid any catastrophes. Let’s practice using assert to get a feel for automated testing!



Small World Air has a program that runs at the check-in kiosk and asks passengers for their destination airport code.

Currently, Small World only flies to three destinations: Budapest (BUD), Casablanca (CMN), and Istanbul (IST). Take some time to examine the program.

What should the program return for the current set destination of 'HND' (an airport in Tokyo, Japan)?


This error wasn’t a very user-friendly experience! We also want to make sure that users are not entering destinations that Small Air World does not travel to. Let’s add an automated test using assert!

Add an assert statement that checks if destination is in the destinations keys. If it isn’t, the AssertionError message should read, 'Sorry, Small World currently does not fly to this destination!'

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