code: def test_no_op(self):
      assert self.df.equals(msno.nullity_filter(self.df))
      assert self.df.equals(msno.nullity_filter(self.df, filter="top"))
      assert self.df.equals(
          msno.nullity_filter(self.df, filter="bottom")
      )
  
imports: import unittest
  import pandas
  import numpy
  import sys
  import missingno
label_desc: Use image node_id 2 for calling the TestNullityFilter obj's underlying member method code with example usage: obj.test_no_op() without return types