code: def get_metric_group_definition(self, group_name):
          
          if group_name not in self._metric_group_defs:
              raise ValueError("A metric group definition with this name does "
                               "not exist: {}".format(group_name))
          return self._metric_group_defs[group_name]
docs: Get a faked metric group definition by its group name.
  
          Parameters:
  
            group_name (:term:`string`): Name of the metric group.
  
          Returns:
  
            :class:~zhmcclient.FakedMetricGroupDefinition`: Definition of the
              metric group.
  
          Raises:
  
            ValueError: A metric group definition with this name does not exist.