code: def set( self, instance, value, **kwargs ):
          
          if not self._allow_write:
              raise AttributeError( "can't set Ref directly, allow_write is disabled" )
          target = instance
          for attr in self._path[:-1]:
              target = getattr( target, attr )
          setattr( target, self._path[-1], value )
          return
docs: Set an attribute on an object using the Ref path.
  
          instance
              The object instance to traverse.
  
          value
              The value to set.
  
          Throws AttributeError if allow_write is False.