code: def get_mock_reader(self) -> DatasetReader:
      class MockReader(DatasetReader):
          def __init__(self, instances, **kwargs):
              super().__init__(**kwargs)
              self.instances = instances
  
          def _read(self, file_path: str):
              for instance in self.instances:
                  yield instance
  
      return MockReader(self.instances)
  
file_path: allennlp/tests/data/samplers/sampler_test.py
imports: from typing import List, Iterable, Dict, Union
  from allennlp.common.testing import AllenNlpTestCase
  from allennlp.data import Vocabulary, Instance, Token, Batch, DatasetReader
  from allennlp.data.fields import TextField
  from allennlp.data.token_indexers import SingleIdTokenIndexer
label_desc: Use image node_id 2 for calling the SamplerTest obj's underlying member method code with example usage: obj.get_mock_reader() and returns: MockReader