Python unittest: Testing an exception is not raised

testcase_assert_not_raises.py

import sys
import unittest
from contextlib import contextmanager


@contextmanager
def assert_not_raises(self, exc_type):
    try:
        yield None
    except exc_type:
        tb = sys.exc_info()[2]
        msg = '{} raised'.format(exc_type.__name__)
        raise self.failureException(msg).with_traceback(tb) from None


unittest.TestCase.assertNotRaises = assert_not_raises


class TestCase(unittest.TestCase):
    def test_it_does_not_raise_key_error(self):
        data = {}
        with self.assertNotRaises(KeyError):
            data['missing key']

    def test_not_raise(self):
        with self.assertNotRaises(Exception):
            pass


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