code: def test_split_to_multiindex_expand_no_splits():
      # https://github.com/pandas-dev/pandas/issues/23677
  
      idx = Index(["nosplit", "alsonosplit", np.nan])
      result = idx.str.split("_", expand=True)
      exp = idx
      tm.assert_index_equal(result, exp)
      assert result.nlevels == 1
  
imports: from datetime import datetime
  import re
  import numpy
  import pytest
  import pandas
  from pandas import DataFrame, Index, MultiIndex, Series, _testing
  from pandas.tests.strings import _convert_na_value, object_pyarrow_numpy
label_desc: Use image node_id 19 for calling a global function with example usage: test_split_to_multiindex_expand_no_splits() without return types