code: def keys(self):
      if self._key_indices is None:
          return self._dataset.keys
      else:
          return tuple(
              self._dataset.keys[key_index]
              for key_index in self._key_indices
          )
  
imports: import numbers
  import numpy
  import six
  from chainer.dataset.tabular import tabular_dataset
label_desc: Use image node_id 3 for calling the _Slice obj's underlying member method code with example usage: obj.keys() and returns: self, tuple