code: def testRunOnceTransitionObserver(
      self, max_steps, max_episodes, expected_steps
  ):
      env = driver_test_utils.PyEnvironmentMock()
      policy = driver_test_utils.PyPolicyMock(
          env.time_step_spec(), env.action_spec()
      )
      transition_replay_buffer_observer = MockReplayBufferObserver()
      driver = py_driver.PyDriver(
          env,
          policy,
          observers=[],
          transition_observers=[transition_replay_buffer_observer],
          max_steps=max_steps,
          max_episodes=max_episodes,
          end_episode_on_boundary=False,
      )
  
      initial_time_step = env.reset()
      initial_policy_state = policy.get_initial_state()
      driver.run(initial_time_step, initial_policy_state)
  
      transitions = transition_replay_buffer_observer.gather_all()
      self.assertLen(transitions, expected_steps)
      # TimeStep, Action, NextTimeStep
      self.assertLen(transitions[0], 3)
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  from absl.testing import parameterized
  import numpy
  import tensorflow
  from tf_agents.drivers import py_driver
  from tf_agents.drivers import test_utils
  from tf_agents.environments import batched_py_environment
  from tf_agents.trajectories import trajectory
label_desc: Use image node_id 3 for calling the PyDriverTest obj's underlying member method code with example usage: obj.testRunOnceTransitionObserver(max_steps, max_episodes, expected_steps) without return types