code: def test_pos_ratio(self):
      if self.pos_exist and self.neg_exist:
          dataset = SiameseDataset(
              self.dataset_0,
              self.dataset_1,
              0.5,
              labels_0=self.labels_0,
              labels_1=self.labels_1,
          )
          for i in range(10):
              example = dataset[i]
              self._check_example(example)
          self.assertEqual(len(dataset), N)
      else:
          with self.assertRaises(ValueError):
              dataset = SiameseDataset(
                  self.dataset_0,
                  self.dataset_1,
                  0.5,
                  labels_0=self.labels_0,
                  labels_1=self.labels_1,
              )
  
file_path: chainercv/tests/datasets_tests/test_siamese_dataset.py
imports: import unittest
  import numpy
  from chainer import testing
  from chainer.datasets import TupleDataset
  from chainercv.datasets import SiameseDataset
  from chainercv.utils.testing.assertions.assert_is_image import assert_is_image
label_desc: Use image node_id 4 for calling the TestSiameseDataset obj's underlying member method code with example usage: obj.test_pos_ratio() without return types