Example of asserting properties of an exception caught by assertRaises / assert_raises. Run with `python -m unittest2 -v exception_attrs` from the same dir as the file.

exception_attrs.py

import unittest2

# some code under test
def raise_msg(x):
    raise ValueError('hello: %s' % x)

class SomeError(ValueError):
    def __init__(self, x):
        self.x = x

def raise_attr(x):
    raise SomeError(x)

# two methods of asserting properties of an exception

class ExampleTest(unittest2.TestCase):
    def test_assert_message(self):
        with self.assertRaises(ValueError) as cm:
            raise_msg(42)
        # passing assertion
        self.assertEqual('hello: 42', cm.exception.args[0])
        # failing assertion
        self.assertEqual('hello: 43', cm.exception.args[0])

    def test_assert_attr(self):
        with self.assertRaises(ValueError) as cm:
            raise_attr(42)
        # passing assertion
        self.assertEqual(cm.exception.x, 42)
        # failing assertion
        self.assertEqual(cm.exception.x, 43)
        

output.txt

(venv)marmida@monolith:~/develop/example-exception-cm$ python -m unittest2 -v fixture1 
test_assert_attr (fixture1.ExampleTest) ... FAIL
test_assert_message (fixture1.ExampleTest) ... FAIL

======================================================================
FAIL: test_assert_attr (fixture1.ExampleTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "fixture1.py", line 31, in test_assert_attr
    self.assertEqual(cm.exception.x, 43)
AssertionError: 42 != 43

======================================================================
FAIL: test_assert_message (fixture1.ExampleTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "fixture1.py", line 23, in test_assert_message
    self.assertEqual('hello: 43', cm.exception.args[0])
AssertionError: 'hello: 43' != 'hello: 42'

----------------------------------------------------------------------
Ran 2 tests in 0.001s

FAILED (failures=2)