class_docstr: A helper mixin for testing Reverb adders.
  
  Note that any test inheriting from this mixin must also inherit from something
  that provides the Python unittest assert methods.
code: def items(self):
      sampler = self.client.sample(
          table=adders.DEFAULT_PRIORITY_TABLE,
          num_samples=self.num_items(),
          emit_timesteps=False,
      )
      return [sample.data for sample in sampler]
  
imports: from typing import Any, Callable, Optional, Sequence, Tuple, TypeVar, Union
  from acme import specs
  from acme import types
  from acme.adders import base
  from acme.adders import reverb
  from acme.utils import tree_utils
  import dm_env
  import numpy
  import reverb
  import tensorflow
  import tree
  from absl.testing import absltest
label_desc: Use image node_id 6 for calling the AdderTestMixin obj's underlying member method code with example usage: obj.items() and returns: unknown