code: def test_difficult(self):
      if not self.return_difficult:
          return
  
      i = np.random.randint(0, len(self.dataset))
      _, bbox, _, difficult = self.dataset[i]
      self.assertIsInstance(difficult, np.ndarray)
      self.assertEqual(difficult.dtype, np.bool)
      self.assertEqual(difficult.shape, (bbox.shape[0],))
  
      if not self.use_difficult:
          np.testing.assert_equal(difficult, 0)
  
file_path: chainercv/tests/datasets_tests/voc_tests/test_voc_bbox_dataset.py
imports: import unittest
  import numpy
  from chainer import testing
  from chainer.testing import attr
  from chainer.testing import condition
  from chainercv.datasets import voc_bbox_label_names
  from chainercv.datasets import VOCBboxDataset
  from chainercv.utils import assert_is_bbox_dataset
label_desc: Use image node_id 3 for calling the TestVOCBboxDataset obj's underlying member method code with example usage: obj.test_difficult() without return types