code: def test_contours_alpha_op(self):
      contours = Contours(
          [
              {("x", "y"): [(0, 0), (0, 1), (1, 0)], "alpha": 0.7},
              {("x", "y"): [(1, 0), (1, 1), (0, 1)], "alpha": 0.3},
          ],
          vdims="alpha",
      ).opts(alpha="alpha")
      msg = 'ValueError: Mapping a dimension to the "alpha" style'
      with pytest.raises(AbbreviatedException, match=msg):
          mpl_renderer.get_plot(contours)
  
imports: import numpy
  import pytest
  from holoviews.core import NdOverlay
  from holoviews.core.options import AbbreviatedException
  from holoviews.core.spaces import HoloMap
  from holoviews.element import Contours, Path, Polygons
  from .test_plot import TestMPLPlot, mpl_renderer
label_desc: Use image node_id 7 for calling the TestContoursPlot obj's underlying member method code with example usage: obj.test_contours_alpha_op() without return types