# Method that does some work
# The interesting data is a part of "param", but it's not directly accessible
def do_work(param):
    # Navigating object graph to get to the array
    data_array = param.related_object.some_dict['data_array']

    # This array contains more data, we remove it
    relevant_array = [x for x in data_array if not useless(x)]

    # Only now we can start processing business logic
    result = [process(x) for x in relevant_array]

    return result

# In tests
def test():
    # Since the "param" is some complex structure,
    # that is probably used for something else,
    # we need to do a lot of effort to mock it

    # Produces more data, which is mostly "useless()", hence it's slower
    data_array = Factory.get_generic_data_array()
    fake_param = MagicMock()
    fake_param.related_object.some_dict['data_array'] = data_array

    # Only now we can finally execute it
    assert do_work(fake_param) == Factory.get_do_work_result()