def GetSelectedContextsAndNodes(self):
        items = self._dag_tree.GetSelections()
        ctxs_and_nodes = []
        for item in items:
            data = self._dag_tree.GetPyData(item)
            if not data:
                return
            ctx, node = data
            if (ctx, node) not in ctxs_and_nodes:
                ctxs_and_nodes.append((ctx, node))
        return ctxs_and_nodes
Testing the `unionagg` (aggregate union) GeoQuerySet method.
dataset/ETHPy150Open manahl/mdf/mdf/viewer/frame.py/MDFViewerFrame.GetSelectedContextsAndNodes--crossover with--dataset/ETHPy150Open AppScale/appscale/AppServer/lib/django-1.5/django/contrib/gis/tests/geoapp/tests.py/GeoQuerySetTest.test_unionagg