Most real-life projects have various interdependencies between components. While testing one component, the result must not be affected by the behavior of other components. For example, your application might call an external web service that might be unreliable in terms of network connection or slow to respond.

Mock objects imitate such dependencies by having the same interface, but they respond to method calls with canned responses. After using a mock object in a test, you can assert whether a certain method was called and verify that the expected interaction took place.

Take the example of the SuperHero profile eligibility test mentioned in Pattern: Service objects (see Chapter 3, Models). We are going to mock the call to the service object method in a test using the Python 3 unittest.mock library:

# profiles/tests.py
from django.test import TestCase
from unittest.mock import patch
from django.contrib.auth.models import User

class TestSuperHeroCheck(TestCase):
    def test_checks_superhero_service_obj(self):
        with patch("profiles.models.SuperHeroWebAPI") as ws:
            ws.is_hero.return_value = True
            u = User.objects.create_user(username="t")
            r = u.profile.is_superhero()
        ws.is_hero.assert_called_with('t')
        self.assertTrue(r)

Here, we are using patch() as a context manager in a with statement. Since the profile model's is_superhero() method will call the SuperHeroWebAPI.is_hero() class method, we need to mock it inside the models module. We are also hard-coding the return value of this method to be True.

The last two assertions check whether the method was called with the correct arguments and if is_hero() returned True, respectively. Since all methods of SuperHeroWebAPI class have been mocked, both the assertions will pass.

Mock objects come from a family called Test Doubles, which includes stubs, fakes, and so on. Like movie doubles who stand in for real actors, these test doubles are used in place of real objects while testing. While there are no clear lines drawn between them, Mock objects are objects that can test the behavior, and stubs are simply placeholder implementations.