code: def test_union_identity(self, index, sort):
      first = index[5:20]
  
      union = first.union(first, sort=sort)
      # i.e. identity is not preserved when sort is True
      assert (union is first) is (not sort)
  
      # This should no longer be the same object, since [] is not consistent,
      # both objects will be recast to dtype('O')
      union = first.union(Index([], dtype=first.dtype), sort=sort)
      assert (union is first) is (not sort)
  
      union = Index([], dtype=first.dtype).union(first, sort=sort)
      assert (union is first) is (not sort)
  
imports: from datetime import datetime
  import operator
  import numpy
  import pytest
  from pandas._libs import lib
  from pandas.core.dtypes.cast import find_common_type
  from pandas import CategoricalDtype, CategoricalIndex, DatetimeTZDtype, Index, MultiIndex, PeriodDtype, RangeIndex, Series, Timestamp
  import pandas._testing
  from pandas.api.types import is_signed_integer_dtype, pandas_dtype
label_desc: Use image node_id 8 for calling the TestSetOpsUnsorted obj's underlying member method code with example usage: obj.test_union_identity(index, sort) without return types