code: def __len__(self):
      return len(self.ids)
  
file_path: chainercv/chainercv/datasets/coco/coco_instances_base_dataset.py
imports: from collections import defaultdict
  import json
  import numpy
  import os
  import PIL.Image
  import PIL.ImageDraw
  from chainercv.chainer_experimental.datasets.sliceable import GetterDataset
  from chainercv.datasets.coco.coco_utils import get_coco
  from chainercv import utils
label_desc: Use image node_id 2 for calling the COCOInstancesBaseDataset obj's underlying member method code with example usage: obj.__len__() and returns: len