code: def keys(self):
      return self._keys
  
file_path: chainer/tests/chainer_tests/dataset_tests/tabular_tests/dummy_dataset.py
imports: import numpy
  import chainer
  from chainer import testing
label_desc: Use image node_id 3 for calling the DummyDataset obj's underlying member method code with example usage: obj.keys() and returns: self