code: def set_observer(self, observer):
          
          observer = str(observer)
          if observer not in color_constants.OBSERVERS:
              raise InvalidObserverError(self)
          self.observer = observer
docs: Validates and sets the color's observer angle.
  
          .. note:: This only changes the observer angle value. It does no conversion
              of the color's coordinates.
  
          :param str observer: One of '2' or '10'.