code: def default(self, meth):
          
          if self._default is not NOTHING:
              raise DefaultAlreadySetError()
  
          self._default = Factory(meth, takes_self=True)
  
          return meth
docs: Decorator that allows to set the default for an attribute.
  
          Returns *meth* unchanged.
  
          :raises DefaultAlreadySetError: If default has been set before.
  
          .. versionadded:: 17.1.0