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

        Subclasses may override this method.

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

        Subclasses may override this method.
        """
        glyph = self.glyph
        if glyph is not None:
            parent = glyph
        else:
            parent = self.font
        if parent is None:
            return None
        return parent.guidelines.index(self)