code: def faces(self):
          
  
          if self._faces is None:
              if self._vertices is None:
                  return None
              self.triangulate()
          return self._faces
docs: Return an array (Nf, 3) of vertex indexes, three per triangular
          face in the mesh.
  
          If faces have not been computed for this mesh, the function
          computes them.
          If no vertices or faces are specified, the function returns None.