code: def __init__(self, ratings):
      self.ratings = ratings
      self.folds = list(
          xf.partition_users(self.ratings, 5, xf.SampleFrac(0.2))
      )
      self.test = pd.concat(f.test for f in self.folds)
      self.isolate = False
  
imports: import pytest
  from collections import namedtuple
  import logging
  import pandas
  import numpy
  import lenskit.util.test
  from lenskit.algorithms.basic import TopN, Popular
  from lenskit.algorithms.bias import Bias
  from lenskit import batch, topn
  import lenskit.crossfold
label_desc: Use image node_id 1 to create a new MLFolds object  with example: obj = MLFolds(ratings)