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You have a class whose API you want to verify:

    class Class_A(object):
        def method_X(self):
            self.method_Y()

        def method_Y(self):
            pass

You write the following test fully expecting it to pass. Alas, the method_Y assertion fails.

    def test_class_a(self):
        obj = Class_A()
        mock = MagicMock(spec_set=obj, wraps=obj)

        mock.method_X()
        mock.method_Y.assert_called_once_with()

This is because the Python Unittest Mock, while wrapping mock.method_X(), Mock uses obj in the __self__ of the method_Y binding. As a result the self value passed to method_X is that of the obj and not mock and the call to method_Y is therefore invisible to the Mock.

Continue reading “How to spy on Mock-wrapped method calls in Python”