<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def train_cv(self, num_folds, fold, random=None):
 """ 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 """

        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))