code: def __len__(self):
      return len(self.values)
  
file_path: chainer/tests/chainer_tests/dataset_tests/test_dataset_mixin.py
imports: import unittest
  import numpy
  from chainer import dataset
  from chainer import testing
label_desc: Use image node_id 2 for calling the SimpleDataset obj's underlying member method code with example usage: obj.__len__() and returns: len