def get_metric_index(self, data_source):
        """
        This function will return the elasticsearch index for a corresponding
        data source. It chooses in between the default and the user inputed
        es indices and returns the user inputed one if it is available.

        :param data_source: the data source for which the index has to be returned
        :returns: an elasticsearch index name
        """

        if data_source in self.index_dict:
            index = self.index_dict[data_source]
        else:
            index = self.class2index[self.ds2class[data_source]]
        return Index(index_name=index)