Mock variable in function

Just use @patch() to mock out get_complex_data_structure():

@patch('module_under_test.get_complex_data_structure')
def test_function_to_test(self, mocked_function):
    foo_mock = mocked_function.return_value

When the test function then calls get_complex_data_structure() a mock object is returned and stored in the local name foo; the very same object that mocked_function.return_value references in the above test; you can use that value to test if do_work() got passed the right object, for example.