def add_layer(self, layer=None):
        layer = layer or core.Data()
        self.widget.data_collection.append(layer)
        return layer
Return a list of tuples, each tuple containing a key/value pair.
dataset/ETHPy150Open glue-viz/glue/glue/app/qt/tests/test_layer_tree_widget.py/TestLayerTree.add_layer--crossover with--dataset/ETHPy150Open RoseOu/flasky/venv/lib/python2.7/site-packages/sqlalchemy/engine/result.py/RowProxy.items