@staticmethod
def addable_cells(obj):
    """
        List object addable cells

        TESTS ::

            sage: from sage.combinat.skew_partition import SkewPartition
            sage: from sage_widget_adapters.combinat.skew_partition_grid_view_adapter import SkewPartitionGridViewAdapter
            sage: sp = SkewPartition([[4, 2, 1],[2, 1]])
            sage: SkewPartitionGridViewAdapter.addable_cells(sp)
            [(0, 1), (1, 0), (0, 4), (1, 2), (2, 1), (3, 0)]
        """
    return obj.inner().corners() + obj.outer().outside_corners()
