code: def test_to_csv_na_rep(self):
      # see gh-11553
      #
      # Testing if NaN values are correctly represented in the index.
      df = DataFrame({"a": [0, np.nan], "b": [0, 1], "c": [2, 3]})
      expected_rows = ["a,b,c", "0.0,0,2", "_,1,3"]
      expected = tm.convert_rows_list_to_csv_str(expected_rows)
  
      assert df.set_index("a").to_csv(na_rep="_") == expected
      assert df.set_index(["a", "b"]).to_csv(na_rep="_") == expected
  
      # now with an index containing only NaNs
      df = DataFrame({"a": np.nan, "b": [0, 1], "c": [2, 3]})
      expected_rows = ["a,b,c", "_,0,2", "_,1,3"]
      expected = tm.convert_rows_list_to_csv_str(expected_rows)
  
      assert df.set_index("a").to_csv(na_rep="_") == expected
      assert df.set_index(["a", "b"]).to_csv(na_rep="_") == expected
  
      # check if na_rep parameter does not break anything when no NaN
      df = DataFrame({"a": 0, "b": [0, 1], "c": [2, 3]})
      expected_rows = ["a,b,c", "0,0,2", "0,1,3"]
      expected = tm.convert_rows_list_to_csv_str(expected_rows)
  
      assert df.set_index("a").to_csv(na_rep="_") == expected
      assert df.set_index(["a", "b"]).to_csv(na_rep="_") == expected
  
      csv = pd.Series(["a", pd.NA, "c"]).to_csv(na_rep="ZZZZZ")
      expected = tm.convert_rows_list_to_csv_str(
          [",0", "0,a", "1,ZZZZZ", "2,c"]
      )
      assert expected == csv
  
imports: import io
  import os
  import sys
  from zipfile import ZipFile
  from _csv import Error
  import numpy
  import pytest
  import pandas
  from pandas import DataFrame, Index, compat
  import pandas._testing
label_desc: Use image node_id 9 for calling the TestToCSV obj's underlying member method code with example usage: obj.test_to_csv_na_rep() without return types