code: def _resolveParam(self, param):
          
          if isinstance(param, Param):
              self._shouldOwn(param)
              return param
          elif isinstance(param, basestring):
              return self.getParam(param)
          else:
              raise ValueError("Cannot resolve %r as a param." % param)
docs: Resolves a param and validates the ownership.
  
          :param param: param name or the param instance, which must
                        belong to this Params instance
          :return: resolved param instance