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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_metric_group_definition(self, group_name):
 """ 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. """

        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]