code: def test_epoch(self):
      reader = get_2d_reader()
      batch_size = 3
      sampler = ImageWindowGenerator(
          reader=reader, batch_size=batch_size, epoch=1
      )
      with self.cached_session() as sess:
          next_element = sampler.pop_batch_op()
          iters = 0
          try:
              for _ in range(400):
                  window = sess.run(next_element)
                  iters = iters + 1
          except tf.errors.OutOfRangeError:
              pass
          # batch size 3, 40 images in total
          self.assertEqual(
              np.ceil(reader.num_subjects / np.float(batch_size)), iters
          )
  
imports: from __future__ import absolute_import, print_function
  import os
  import unittest
  import numpy
  import tensorflow
  from niftynet.engine.image_window import N_SPATIAL, LOCATION_FORMAT
  from niftynet.engine.image_window_dataset import ImageWindowDataset
  from niftynet.io.image_reader import ImageReader
  from tests.niftynet_testcase import NiftyNetTestCase
label_desc: Use image node_id 8 for calling the ImageWindowDataset_Generator_2D_Test obj's underlying member method code with example usage: obj.test_epoch() without return types