Get the point's index.
        This must return an ``int``.

        Subclasses may override this method.

def _get_index(self):
        """
        Get the point's index.
        This must return an ``int``.

        Subclasses may override this method.
        """
        contour = self.contour
        if contour is None:
            return None
        return contour.points.index(self)