Question

I have the following test-code checking for an exception raising in a function. I expect the test to pass, but a failure is indicated instead. Here is the test code:

import unittest

# define a user-defined exception
class MyException(Exception):
    def __str__(self):
        return repr("ERROR: Just raised my exception!")

# this is my main class with a method raising this exception
class MyMainObject(object):

    def func(self):
        raise MyException()

# the test class
class TestConfig(unittest.TestCase):

    def test_1(self):

        other = MyMainObject()
        self.assertRaises(MyException, other.func())

# calling the test
if __name__ == '__main__':    
    unittest.main()

When other.func() is called in the assert statement, MyException is raised (can be checked easily). So, the assertRaises test should pass the test, as other.func() failes with MyException, BUT:

....
MyException: 'ERROR: Just raised my exception!'

----------------------------------------------------------------------
Ran 1 test in 0.001s

FAILED (errors=1)

I do not see something wrong, so I would appreciate some input on this problem.

Answer 1

assertRaises calls the function for you. By calling it yourself, the exception is raised before assertRaises can test it.

Change your code to:

self.assertRaises(MyException, other.func)

and it’ll work correctly. Alternatively, you can use assertRaises as a context manager (python 2.7 and up):

with self.assertRaises(MyException):
    other.func()

Using assertRaises as a context manager has the added advantage that you can now retrieve the exception instance and perform further tests on it:

with self.assertRaises(MyException) as raises_cm:
    other.func()

exception = raises_cm.exception
self.assertEqual(exception.args, ('foo', 'bar'))