Python unittest: Testing an exception is not raised

testcase_assert_not_raises.py

import unittest
from contextlib import contextmanager


class TestCase(unittes.TestCase):

    @contextmanager
    def assertNotRaises(self, exc_type):
        try:
            yield None
        except exc_type:
            raise self.failureException('{} raised'.format(exc_type.__name__))
            
    def test_it_does_not_raise_key_error(self):
        data = {}
        
        with self.assertNotRaise(KeyError):
            data['missing key']

There's a typo -> class TestCase(unittes.TestCase):

unittes should be unittest.

And a second typo in the with self ... statement ... the assertNotRaise misses the trailing s too.