Python – Unit Testing an Impure Function with Method Calls

I wish to test the following method:

class MyClass(object):
    ...
    def my_method(self, args):
        ...  # Code which transforms args into arg1, arg2, ..., argN
        self.A.other_method1(arg1, arg2)
        self.A.other_method2(arg1, arg2, arg3)
        self.A.other_method3(arg1)
        self.B.other_method4(arg1, arg2)
        self.B.other_method5(arg1, arg2, arg4)
        self.B.other_method6(arg1, arg2, arg5, arg6)

Obviously this is an artificial example. I have created this example to focus on what I believe are the essential points:

• my_method first does some straightforward work to transform the values it is given. I am not concerned with how to test this aspect.
• my_method then calls a sequence of methods on other attributes A and B of the class. I am not concerned with how to test those other methods.
• The sequence of method calls mutates A and B. So my_method is very much a non-pure function. I don't believe I can redesign to make the function pure, as the other methods all do I/O.

My current approach is to mock out A and B using unittest.mock.MagicMock, and then assert that these methods were called in the required order, with the expected arguments. My test code therefore looks like this:

from unittest.mock import MagicMock, call

class MyTest(unittest.TestCase):
    def test_MyClass_my_method(self):
        x = MyClass()  # Instantiation details omitted
        x.A = MagicMock()
        x.B = MagicMock()
        x.my_method(0)
        result_A = x.A.method_calls
        result_B = x.B.method_calls
        expected_result_A = [call.other_method1(1, 2),
                             call.other_method2(1, 2, 3),
                             call.other_method2(1)]
        expected_result_B = [call.other_method4(1, 2),
                             call.other_method5(1, 2, 4),
                             call.other_method6(1, 2, 5, 6)]

        self.assertEquals(result_A, expected_result_A)
        self.assertEquals(result_B, expected_result_B)

This works. But I can't help feel I have simply rewritten my_method – and, worse, tightly coupled the test to the inner working of the method.

Is there a better way to test this method? Is the method actually poorly designed?