code: def get_value_ddist(self, attr_name, attr_value):
          
          assert not self.tree.data.is_continuous_class, \
              "Discrete distributions are only maintained for " + \
              "discrete class types."
          ddist = DDist()
          cls_counts = self._attr_class_value_counts[attr_name][attr_value]
          for cls_value, cls_count in iteritems(cls_counts):
              ddist.add(cls_value, count=cls_count)
          return ddist
docs: Returns the class value probability distribution of the given
          attribute value.