Question

In the Python unittest framework, is there a way to pass a unit test if an exception wasn’t raised, and fail with an AssertRaise otherwise?

Answer 1

If I understand your question correctly, you could do something like this:

def test_does_not_raise_on_valid_input(self):
    raised = False
    try:
        do_something(42)
    except:
        raised = True
    self.assertFalse(raised, 'Exception raised')

…assuming that you have a corresponding test that the correct Exception gets raised on invalid input, of course:

def test_does_raise_on_invalid_input(self):
    self.assertRaises(OutOfCheese, do_something, 43)

However, as pointed out in the comments, you need to consider what it is that you are actually testing. It’s likely that a test like…

def test_what_is_42(self):
    self.assertEquals(do_something(42), 'Meaning of life')

…is better because it tests the desired behaviour of the system and will fail if an exception is raised.