class_docstr: A derived class from :class:`Dataset` for which folds (for
  cross-validation) are predefined.
code: def __init__(self, folds_files=None, reader=None):
      Dataset.__init__(self, reader)
      self.folds_files = folds_files
  
      # check that all files actually exist.
      for train_test_files in self.folds_files:
          for f in train_test_files:
              if not os.path.isfile(os.path.expanduser(f)):
                  raise ValueError(
                      "File " + str(f) + " does not exist."
                  )
  
imports: import itertools
  import os
  import sys
  from collections import defaultdict
  from .builtin_datasets import BUILTIN_DATASETS, download_builtin_dataset
  from .reader import Reader
  from .trainset import Trainset
label_desc: Use image node_id 1 to create a new DatasetUserFolds object from inherited base classes: Dataset with example: obj = DatasetUserFolds(folds_files, reader)