code: def set_missing_value_policy(self, policy, target_attr_name=None):
          
          assert policy in MISSING_VALUE_POLICIES, \
              "Unknown policy: %s" % (policy,)
          for attr_name in self.data.attribute_names:
              if target_attr_name is not None and target_attr_name != attr_name:
                  continue
              self.missing_value_policy[attr_name] = policy
docs: Sets the behavior for one or all attributes to use when traversing the
          tree using a query vector and it encounters a branch that does not
          exist.