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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 getDataset(self, id_):
 """ Returns a dataset with the specified ID, or raises a DatasetNotFoundException if it does not exist. """

        if id_ not in self._datasetIdMap:
            raise exceptions.DatasetNotFoundException(id_)
        return self._datasetIdMap[id_]