I am looking into the excellent pytest plugin called pytest-mock (https://github.com/pytest-dev/pytest-mock), and now I am trying some examples with assert_has_calls. In short I am testing an instance of class B, more specifically how that instance interact with an instance of class A (in which I have mocked the method 'time_consuming_task').
The example is working with alt. B (see comments in code). I would prefer alt. A, and instead mock the method in class A directly, instead of mocking the method in the instance of class A accessed through the instance (obj) of class B.
class A(object):
    def do_time_consuming_task(self, timeout):
        return True
class B(object):
    def __init__(self):
        self.a = A()
    def do_work(self, timeout):
        return self.a.do_time_consuming_task(timeout)
def test_calls(mocker):
    # Prepare
    obj = B()
    #mock_a = mocker.patch.object(A, 'do_time_consuming_task', autospec=True)  # Alt. A
    mock_a = mocker.patch.object(obj.a, 'do_time_consuming_task', autospec=True)  # Alt. B
    mock_a.return_value = True
    # Exercise
    obj.do_work(timeout=100)
    obj.do_work(timeout=50)
    # Assert
    mock_a.assert_has_calls([mocker.call(100), mocker.call(50)])