def test_attach(self):
        """
        Test the attach method.

        @raise AssertionError: If the test fails.
        """
        observable = self.observable_class()
        observer_1 = self.observer_class()
        observer_2 = self.observer_class()
        observer_3 = self.observer_class()

        observable.attach(observer_1)
        observable.attach(observer_2)
        observable.attach(observer_3)

        try:
            observable.attach(observer_1)
        except:
            raise AssertionError
        else:
            self.assertEquals({observer_1, observer_2, observer_3}, observable._observers)

        Test the attach method.

        @raise AssertionError: If the test fails.
        
tests/behavioral_tests/test_observer.py