code: def test_block_attr_get_item_(multiblock_poly):
      """Verify flat indexing is correct."""
      block_b = multiblock_poly.copy()
      block_c = multiblock_poly.copy()
      block_a = pv.MultiBlock([block_b, block_c])
      pl = pv.Plotter()
      actor, mapper = pl.add_composite(block_a)
      assert len(mapper.block_attr) == len(block_b) + len(block_c) + 3
      with pytest.raises(KeyError):
          mapper.block_attr[len(mapper.block_attr)]
  
imports: import pytest
  from vtkmodules.vtkCommonCore import vtkLookupTable
  import pyvista
  from pyvista.plotting.composite_mapper import BlockAttributes, CompositePolyDataMapper
label_desc: Use image node_id 13 for calling a global function with example usage: test_block_attr_get_item_(multiblock_poly) without return types