code: def set_helper(self, helper):
          
          # We don't want to "unset" in this method.
          if helper is None:
              return
  
          # Get the right kind of helper if given a DockerHelper
          if isinstance(helper, DockerHelper):
              helper = helper._helper_for_model(self.__model_type__)
  
          # We already have this one.
          if helper is self._helper:
              return
          if self._helper is None:
              self._helper = helper
          else:
              raise RuntimeError('Cannot replace existing helper.')
docs: .. todo::
  
              Document this.