code: def test_ravel(index):
      # GH#19956 ravel returning ndarray is deprecated, in 2.0 returns a view on self
      res = index.ravel()
      tm.assert_index_equal(res, index)
  
imports: import re
  import numpy
  import pytest
  from pandas.errors import InvalidIndexError
  import pandas._testing
label_desc: Use image node_id 8 for calling a global function with example usage: test_ravel(index) without return types