Testing Exceptions

Programs throw errors when for instance wrong input is given. Because of this, one needs to make sure that an error is thrown when actual wrong input is given. Because of that we need to check for an exact exception, for this example we will use the following exception:

class WrongInputException(Exception):
    pass

This exception is raised when wrong input is given, in the following context where we always expect a number as text input.

def convert2number(random_input):
    try:
        my_input = int(random_input)
    except ValueError:
        raise WrongInputException("Expected an integer!")
    return my_input

To check whether an exception has been raised, we use assertRaises to check for that exception. assertRaises can be used in two ways:

1. Using the regular function call. The first argument takes the exception type, second a callable (usually a function) and the rest of arguments are passed to this callable.
2. Using a with clause, giving only the exception type to the function. This has as advantage that more code can be executed, but should be used with care since multiple functions can use the same exception which can be problematic. An example:

with self.assertRaises(WrongInputException):
    convert2number("not a number")

This first has been implemented in the following test case:

import unittest

class ExceptionTestCase(unittest.TestCase):

    def test_wrong_input_string(self):
        self.assertRaises(WrongInputException, convert2number, "not a number")

    def test_correct_input(self):
        try:
            result = convert2number("56")
            self.assertIsInstance(result, int)
        except WrongInputException:
            self.fail()

There also may be a need to check for an exception which should not have been thrown. However, a test will automatically fail when an exception is thrown and thus may not be necessary at all. Just to show the options, the second test method shows a case on how one can check for an exception not to be thrown. Basically, this is catching the exception and then failing the test using the fail method.