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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 getVariantSetByName(self, name):
 """ Returns a VariantSet with the specified name, or raises a VariantSetNameNotFoundException if it does not exist. """

        if name not in self._variantSetNameMap:
            raise exceptions.VariantSetNameNotFoundException(name)
        return self._variantSetNameMap[name]