Python | Exceptional Conditions Testing in Unit Tests

This article aims to write a unit test that cleanly tests if an exception is raised. To test for exceptions, the assertRaises() method is used.

Code #1 : Testing that a function raised a ValueError exception

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import unittest
  
# A simple function to illustrate
def parse_int(s):
    return int(s)
  
class TestConversion(unittest.TestCase):
    def test_bad_int(self):
        self.assertRaises(ValueError, parse_int, 'N/A')

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Code #2 : Test the exception’s value with a different approach is needed

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import errno
  
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')

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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 exceptions on their own.

Code #3 : Example

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class TestConversion(unittest.TestCase):
    def test_bad_int(self):
        try:
            r = parse_int('N/A')
        except ValueError as e:
            self.assertEqual(type(e), ValueError)

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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, an extra check for that situation needs to be added, as shown in the code below.

Code #4 :

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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')

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The assertRaises() method simply takes care of these details, so it is preferred to be used. 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, it has to be manually tested. Somewhere in between these two extremes, the assertRaisesRegex() method can be used and it allows to test for an exception and perform a regular expression match against the exception’s string representation at the same time.

Code #5 :

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class TestConversion(unittest.TestCase):
    def test_bad_int(self):
        self.assertRaisesRegex(
                ValueError, 'invalid literal .*'
                parse_int, 'N/A')

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A little-known fact about assertRaises() and assertRaisesRegex() is that they can also be used as context managers.

Code #6 :

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class TestConversion(unittest.TestCase):
    def test_bad_int(self):
        with self.assertRaisesRegex(ValueError, 'invalid literal .*'):
            r = parse_int('N/A')

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This form can be useful if the test involves multiple steps (e.g., setup) besides that of simply executing a callable.



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