code: def _get_edges(self):
          
          if self._edges is None:
              self._edges = self._compute_edges()
          return self._edges
docs: Get the edges for the current surface.
  
          If they haven't been computed yet, first compute and store them.
  
          This is provided as a means for internal calls to get the edges
          without copying (since :attr:`.edges` copies before giving to
          a user to keep the stored data immutable).
  
          Returns:
              Tuple[~bezier.curve.Curve, ~bezier.curve.Curve, \
                    ~bezier.curve.Curve]: The edges of
              the surface.