How do you test that a Python function throws an exception?

Use TestCase.assertRaises from the unittest module, for example:

import mymod

class MyTestCase(unittest.TestCase):
    def test1(self):
        self.assertRaises(SomeCoolException, mymod.myfunc)

Since Python 2.7 you can use context manager to get ahold of the actual Exception object thrown:

import unittest

def broken_function():
    raise Exception('This is broken')

class MyTestCase(unittest.TestCase):
    def test(self):
        with self.assertRaises(Exception) as context:
            broken_function()

        self.assertTrue('This is broken' in context.exception)

if __name__ == '__main__':
    unittest.main()

assertRaises

In Python 3.5, you have to wrap context.exception in str, otherwise you'll get a TypeError

self.assertTrue('This is broken' in str(context.exception))

The code in my previous answer can be simplified to:

def test_afunction_throws_exception(self):
    self.assertRaises(ExpectedException, afunction)

And if a function takes arguments, just pass them into assertRaises like this:

def test_afunction_throws_exception(self):
    self.assertRaises(ExpectedException, afunction, arg1, arg2)

Your code should follow this pattern (this is a unittest module style test):

def test_afunction_throws_exception(self):
    try:
        afunction()
    except ExpectedException:
        pass
    except Exception:
       self.fail('unexpected exception raised')
    else:
       self.fail('ExpectedException not raised')

On Python < 2.7 this construct is useful for checking for specific values in the expected exception. The unittest function assertRaises only checks if an exception was raised.