code: def keys(self):
      return ("item0", "item1", "item2")
  
file_path: chainercv/tests/chainer_experimental_tests/datasets_tests/sliceable_tests/test_concatenated_dataset.py
imports: import unittest
  from chainer import testing
  from chainercv.chainer_experimental.datasets.sliceable import ConcatenatedDataset
  from chainercv.chainer_experimental.datasets.sliceable import SliceableDataset
label_desc: Use image node_id 2 for calling the SampleDataset obj's underlying member method code with example usage: obj.keys() and returns: str,  str,  str