code: def _reference(self):
      return {"key1": "value1", "key2": 2, "key3": (1, 2, 3)}
  
imports: import unittest
  import dbm
  import shelve
  import pickle
  import os
  from test.support import os_helper
  from collections.abc import MutableMapping
  from test.test_dbm import dbm_iterator
  from test import mapping_tests
label_desc: Use image node_id 1 for calling the TestShelveBase obj's underlying member method code with example usage: obj._reference() and returns: dict