code: def test_step_fn(self, mock_learner):
      num_calls = 0
  
      def step_fn():
          nonlocal num_calls
          num_calls += 1
  
      learner = frozen_learner.FrozenLearner(
          mock_learner, step_fn=step_fn
      )
  
      # Step two times.
      learner.step()
      learner.step()
  
      self.assertEqual(num_calls, 2)
      # step() method of the wrapped learner should not be called.
      mock_learner.step.assert_not_called()
  
file_path: acme/acme/utils/frozen_learner_test.py
imports: from unittest import mock
  import acme
  from acme.utils import frozen_learner
  from absl.testing import absltest
label_desc: Use image node_id 1 for calling the FrozenLearnerTest obj's underlying member method code with example usage: obj.test_step_fn(mock_learner) without return types