code: def train_cv(self, num_folds, fold, random=None):
          
          if random is None:
              return Instances(
                  javabridge.call(self.jobject, "trainCV", "(II)Lweka/core/Instances;",
                                  num_folds, fold))
          else:
              return Instances(
                  javabridge.call(self.jobject, "trainCV", "(IILjava/util/Random;)Lweka/core/Instances;",
                                  num_folds, fold, random.jobject))
docs: Generates a training fold for cross-validation.
  
          :param num_folds: the number of folds of cross-validation, eg 10
          :type num_folds: int
          :param fold: the current fold (0-based)
          :type fold: int
          :param random: the random number generator
          :type random: Random
          :return: the training fold
          :rtype: Instances