code: def test_lower_upper(any_string_dtype):
      s = Series(["om", np.nan, "nom", "nom"], dtype=any_string_dtype)
  
      result = s.str.upper()
      expected = Series(
          ["OM", np.nan, "NOM", "NOM"], dtype=any_string_dtype
      )
      tm.assert_series_equal(result, expected)
  
      result = result.str.lower()
      tm.assert_series_equal(result, s)
  
imports: from datetime import datetime
  import operator
  import numpy
  import pytest
  from pandas import Series, _testing
label_desc: Use image node_id 3 for calling a global function with example usage: test_lower_upper(any_string_dtype) without return types