def __missing__(self, key):
        if self.default_factory is None:
            raise KeyError(key)
        else:
            ret = self[key] = self.default_factory(name=key)
            return ret
def test_optional_arguments(self):
        self.assertEqual(self.myfunc('a'), self.expected)
        self.assertEqual(self.myfunc('a', 5), self.expected)
        self.assertEqual(self.myfunc('a', second=5), self.expected)
        self.assertEqual(self.myfunc('a', 5, 5), self.expected)
        self.assertEqual(self.myfunc('a', fourth=[]), self.expected)