code: def test_idxmapping_ndims(self):
      dims = [self.dim1, self.dim2, "strdim"]
      idxmap = MultiDimensionalMapping(kdims=dims)
      self.assertEqual(idxmap.ndims, len(dims))
  
imports: import numpy
  from holoviews import Dataset, HoloMap
  from holoviews.core import Dimension
  from holoviews.core.ndmapping import MultiDimensionalMapping, NdMapping, UniformNdMapping
  from holoviews.core.overlay import Overlay
  from holoviews.element import Curve
  from holoviews.element.comparison import ComparisonTestCase
label_desc: Use image node_id 9 for calling the NdIndexableMappingTest obj's underlying member method code with example usage: obj.test_idxmapping_ndims() without return types