How to mock a class which has been used inside of lets say login class?

I have a login method that I have written a unittest for it and it works great. But login method calls external classes to verify username and password. As I want to make the procedure faster and do the test in isolation I want to mock those classes.

def test_login(self):
        with self.app.test_client() as client:
            response = client.post('/login',
                                   data=json.dumps(dict({'username': 'username',
                                                         'password': 'password'})),
                                   content_type='application/json',
                                   headers={'x-header':'custom headers'})
            self.assertEqual(response.status_code, 200, response.data)

Inside of login there is a class that connects to DB. How should I mock the part conneting to DB? Lets say the DB class is called externalService(). Any thoughts?