code: def _calculate_normals(self):
      vertices, faces = self._vertices, self._faces
      if faces is None:
          # ensure it's always 2D so we can use our methods
          faces = np.arange(0, vertices.size, dtype=np.uint32)[
              :, np.newaxis
          ]
      normals = _calculate_normals(vertices, faces)
      return normals
  
imports: import numpy
  import time
  from gzip import GzipFile
  from os import path
  from ..geometry import _calculate_normals
  from ..util import logger
label_desc: Use image node_id 7 for calling the WavefrontReader obj's underlying member method code with example usage: obj._calculate_normals() and returns: normals