docs: def _get_style_id_from_style(self, style, style_type):
          """
          Return the id of *style*, or |None| if it is the default style of
          *style_type*. Raises |ValueError| if style is not of *style_type*.
          """
          if style.type != style_type:
              raise ValueError(
                  "assigned style is type %s, need type %s" %
                  (style.type, style_type)
              )
          if style == self.default(style_type):
              return None
          return style.style_id