code: def __len__(self):
      return len(self.lists_of_data[0])
  
file_path: dgl/examples/pytorch/rrn/sudoku_data.py
imports: import csv
  import os
  import urllib.request
  import zipfile
  from copy import copy
  import dgl
  import numpy
  import torch
  from torch.utils.data import DataLoader, RandomSampler, SequentialSampler
  from torch.utils.data.dataset import Dataset
label_desc: Use image node_id 3 for calling the ListDataset obj's underlying member method code with example usage: obj.__len__() and returns: len