code: def keys(self):
      return ("item0", "item1", "item2")
  
imports: import numpy
  import six
  import unittest
  from chainercv.chainer_experimental.datasets.sliceable import SliceableDataset
  from chainercv.utils import testing
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