code: def keys(self):
      return tuple(
          key for dataset in self._datasets for key in dataset.keys
      )
  
file_path: chainer/chainer/dataset/tabular/_join.py
imports: import six
  from chainer.dataset.tabular import tabular_dataset
label_desc: Use image node_id 3 for calling the _Join obj's underlying member method code with example usage: obj.keys() and returns: tuple