code: def test_PredifinedKFold():
      reader = Reader(
          line_format="user item rating",
          sep=" ",
          skip_lines=3,
          rating_scale=(1, 5),
      )
  
      current_dir = os.path.dirname(os.path.realpath(__file__))
      folds_files = [
          (current_dir + "/custom_train", current_dir + "/custom_test")
      ]
  
      data = Dataset.load_from_folds(
          folds_files=folds_files, reader=reader
      )
  
      # Make sure rating files are read correctly
      pkf = PredefinedKFold()
      trainset, testset = next(pkf.split(data))
      assert trainset.n_ratings == 6
      assert len(testset) == 3
  
file_path: Surprise/tests/test_split.py
imports: import os
  from collections import Counter
  from copy import copy
  import numpy
  import pytest
  from surprise import Dataset, Reader
  from surprise.model_selection import KFold, LeaveOneOut, PredefinedKFold, RepeatedKFold, ShuffleSplit, train_test_split
  from surprise.model_selection.split import get_cv
label_desc: Use image node_id 6 for calling a global function with example usage: test_PredifinedKFold() without return types