code: def test_sort_values_key_string(self):
      df = DataFrame(
          np.array([["hello", "goodbye"], ["hello", "Hello"]])
      )
  
      result = df.sort_values(1)
      expected = df[::-1]
      tm.assert_frame_equal(result, expected)
  
      result = df.sort_values([0, 1], key=lambda col: col.str.lower())
      tm.assert_frame_equal(result, df)
  
      result = df.sort_values(
          [0, 1], key=lambda col: col.str.lower(), ascending=False
      )
      expected = df.sort_values(
          1, key=lambda col: col.str.lower(), ascending=False
      )
      tm.assert_frame_equal(result, expected)
  
imports: import numpy
  import pytest
  import pandas
  from pandas import Categorical, DataFrame, NaT, Timestamp, date_range
  import pandas._testing
  from pandas.util.version import Version
label_desc: Use image node_id 5 for calling the TestDataFrameSortKey obj's underlying member method code with example usage: obj.test_sort_values_key_string() without return types