code: def test_descending_sort(self):
      result = msno.nullity_sort(self.df, sort="descending")
      expected = self.df.iloc[[0, 1, 2]]
      assert result.equals(expected)
  
imports: import unittest
  import pandas
  import numpy
  import sys
  import missingno
label_desc: Use image node_id 4 for calling the TestNullitySort obj's underlying member method code with example usage: obj.test_descending_sort() without return types