code: def test_sort_values_key_nan(self):
      series = Series(np.array([0, 5, np.nan, 3, 2, np.nan]))
  
      result = series.sort_values(axis=0)
      expected = series.iloc[[0, 4, 3, 1, 2, 5]]
      tm.assert_series_equal(result, expected)
  
      result = series.sort_values(axis=0, key=lambda x: x + 5)
      expected = series.iloc[[0, 4, 3, 1, 2, 5]]
      tm.assert_series_equal(result, expected)
  
      result = series.sort_values(
          axis=0, key=lambda x: -x, ascending=False
      )
      expected = series.iloc[[0, 4, 3, 1, 2, 5]]
      tm.assert_series_equal(result, expected)
  
file_path: pandas/pandas/tests/series/methods/test_sort_values.py
imports: import numpy
  import pytest
  from pandas import Categorical, DataFrame, Series
  import pandas._testing
label_desc: Use image node_id 2 for calling the TestSeriesSortingKey obj's underlying member method code with example usage: obj.test_sort_values_key_nan() without return types