Mocking a return value in a sub object

I want to write a test case for a feature within complex data structures. The feature doesn't rely on all the data and getting real instances with the desired properties is hard. Hence, I am using Mocks.

def test_Case:
    t1 = Timeseries(...) # Data
    t2 = Timeseries(...) # Data

    fancy_t1 = Mock(data=d1, additional_property= ...)
    fancy_t2 = Mock(data=d2, additional_property= ...)

    container = Mock(data_sets=[fancy_t1, fancy_t2])

    ret = function_to_test(container)
    assert ret ... # Some condition

Within function_to_test there is a call of the form

container.aggregation.aggregate(fancy_t, more_arguments1, more_arguments2, ... )

What I want that the aggregation.aggregate call to do is quite simple. It is supposed to evaluate to d1+d2 and ignore the fancy_t and the other arguments.

How would I do that?

If I do something like this

agg = Mock()
agg.return_value = d1 + d2
container.aggregation.get_aggregated_positions = agg

It evalutes to something like <Mock name='mock.aggregation.get_aggregated_positions()' id='296569680'> instead of a Timeseries