code: def setUp(self):
      self.df = pd.DataFrame(
          {
              "A": [0, np.nan, np.nan],
              "B": [0, 0, np.nan],
              "C": [0, 0, 0],
          }
      )
  
imports: import unittest
  import pandas
  import numpy
  import sys
  import missingno
label_desc: Use image node_id 1 for calling the TestNullityFilter obj's underlying member method code with example usage: obj.setUp() without return types