To test for exceptions, use the assertRaises() method. For example, if you want to test

that a function raised a ValueError exception, use this code:

# A simple function to illustrate

def parse_int(s):

return int(s)

def test_bad_int(self):

self.assertRaises(ValueError, parse_int, 'N/A')

If you need to test the exception’s value in some way, then a different approach is needed.

For example:

class TestIO(unittest.TestCase):

def test_file_not_found(self):

try:

f = open('/file/not/found')

except IOError as e:

self.assertEqual(e.errno, errno.ENOENT)

else:

self.fail('IOError not raised')

The assertRaises() method provides a convenient way to test for the presence of an

exception. A common pitfall is to write tests that manually try to do things with excep‐

tions on their own. For instance:

class TestConversion(unittest.TestCase):

def test_bad_int(self):

try:

r = parse_int('N/A')

except ValueError as e:

self.assertEqual(type(e), ValueError)

The problem with such approaches is that it is easy to forget about corner cases, such

as that when no exception is raised at all. To do that, you need to add an extra check for

that situation, as shown here:

class TestConversion(unittest.TestCase):

def test_bad_int(self):

try:

r = parse_int('N/A')

except ValueError as e:

self.assertEqual(type(e), ValueError)

else:

self.fail('ValueError not raised')

The assertRaises() method simply takes care of these details, so you should prefer to

use it.

The one limitation of assertRaises() is that it doesn’t provide a means for testing the

value of the exception object that’s created. To do that, you have to manually test it, as

shown. Somewhere in between these two extremes, you might consider using the as

sertRaisesRegex() method, which allows you to test for an exception and perform a

regular expression match against the exception’s string representation at the same time.

For example:

class TestConversion(unittest.TestCase):

def test_bad_int(self):

self.assertRaisesRegex(ValueError, 'invalid literal .*',

parse_int, 'N/A')

A little-known fact about assertRaises() and assertRaisesRegex() is that they can

also be used as context managers:

class TestConversion(unittest.TestCase):

def test_bad_int(self):

with self.assertRaisesRegex(ValueError, 'invalid literal .*'):

r = parse_int('N/A')

This form can be useful if your test involves multiple steps (e.g., setup) besides that of

simply executing a callable.