human
Solve the following python programming problem given the following code:
@ mock.patch ('nova_solverscheduler.scheduler.solvers.utils.aggregate_values_from_key') 
def test_get_constraint_matrix(self, agg_mock) : 
    self.flags (max_instances_per_host = 1) 
    def _agg_mock_side_effect(* args, **kwargs) : 
        if (args [0].host == 'host1') : 
            return set (['2', '3']) 
if (args [0].host == 'host2') : 
            return set (['4']) 
if (args [0].host == 'host3') : 
            return set ([]) 
if (args [0].host == 'host4') : 
            return set (['invalidval']) 
agg_mock.side_effect = <MASK> 
    expected_cons_mat = [[True, True, False], [True, True, True], [True, False, False], [True, False, False]] 
    cons_mat = self.constraint_cls ().get_constraint_matrix (self.fake_hosts, self.fake_filter_properties) 
    agg_mock.assert_any_call (self.fake_hosts [0], 'max_instances_per_host') 
    agg_mock.assert_any_call (self.fake_hosts [1], 'max_instances_per_host') 
    agg_mock.assert_any_call (self.fake_hosts [2], 'max_instances_per_host') 
    agg_mock.assert_any_call (self.fake_hosts [3], 'max_instances_per_host') 
    self.assertEqual (expected_cons_mat, cons_mat) 
    
    Given the code above, what is a proper replacement for <MASK>? Choose among: self, agg_mock, expected_cons_mat, cons_mat
assistant
#Here is a proper replacement for <MASK>.
def _agg_mock_side_effect(