There is another group of assert methods related to exceptions and warnings. Note that while we haven’t covered warnings in detail yet, they are a type of exception. Let’s go over two of these methods and their general syntax.
assertRaises: The
assertRaises()method takes an exception type as its first argument, a function reference as its second, and an arbitrary number of arguments as the rest.It calls the function and checks if an exception is raised as a result. The test passes if an exception is raised, is an error if another exception is raised, or fails if no exception is raised. This method can be used with custom exceptions as well!
self.assertRaises(specificException, function, functionArguments...)assertWarns: The
assertWarns()method takes a warning type as its first argument, a function reference as its second, and an arbitrary number of arguments for the rest.It calls the function and checks that the warning occurs. The test passes if a warning is triggered and fails if it isn’t.
self.assertWarns(specificWarningException, function, functionArguments...)
There are no particular concise ways to replicate these tests using the assert keyword so it is recommended to use these methods instead when possible!
The full list of exception and warning methods can be seen in the Python documentation. Let’s put these methods into practice!
Instructions
We need to create some tests for the airplane alert system so that the flight crew is properly notified of critical events.
Let’s start by creating a class called SystemAlertTests which inherits from unittest.TestCase.
We are going to create a test for any power outages that might occur on the airplane. Check out the custom exception PowerError and our function that raises the error power_outage_detected() in the alerts.py file. The file is already imported into tests.py for us.
In our SystemAlertTests class, create a test method called test_power_outage_alert().
Inside the new method, use self.assertRaises() to check that an alerts.PowerError is raised whenever
alerts.power_outage_detected is called with an argument of True.
This test should pass since we are passing a value of True and the exception is raised.
We are going to create a test for any water level warnings that occur on the airplane. Check out the custom exception WaterLevelWarning and our function that raises the warning water_levels_check() in the alerts.py file. The file is already imported into tests.py for us.
In our SystemAlertTests class, create a test method called test_water_levels_warning().
Inside the new method, use self.assertWarns() to check that an alerts.WaterLevelWarning is raised whenever alerts.water_levels_check is called with an argument of 150 liters.
This test should pass since we are passing a value less than 200 and a warning occurs.