docstr: Return a dictionary of values which are invariant by storage
  in the object under test.
docstr_tok: ["Return","a","dictionary","of","values","which","are","invariant","by","storage","in","the","object","under","test","."]
code: def _reference(self):
      """Return a dictionary of values which are invariant by storage
      in the object under test."""
      return {"1": "2", "key1": "value1", "key2": (1, 2, 3)}
  
imports: import unittest
  import collections
  from test.support import Py_C_RECURSION_LIMIT
label_desc: Use image node_id 1 for calling the BasicTestMappingProtocol obj's underlying member method code with example usage: obj._reference() and returns: dict