code: def __len__(self):
      return len(self.base)
  
file_path: chainer/examples/imagenet/dataset_util.py
imports: import random
  import chainer
  from chainer import dataset
  from chainer import datasets
label_desc: Use image node_id 2 for calling the PreprocessedDataset obj's underlying member method code with example usage: obj.__len__() and returns: len