code: def test_activations(self):
      (x_train, _), (_, _), (_, _) = self.mnist
      activations = self.defence._get_activations()
      self.assertEqual(len(x_train), len(activations))
  
imports: from __future__ import absolute_import, division, print_function, unicode_literals
  import logging
  import unittest
  from keras.preprocessing.image import ImageDataGenerator
  import numpy
  from art.data_generators import KerasDataGenerator
  from art.defences.detector.poison import ActivationDefence
  from art.utils import load_mnist
  from art.visualization import convert_to_rgb
  from tests.utils import master_seed
label_desc: Use image node_id 8 for calling the TestActivationDefence obj's underlying member method code with example usage: obj.test_activations() without return types