code: def test_to_csv_na_rep_long_string(self, df_new_type):
      # see gh-25099
      df = DataFrame({"c": [float("nan")] * 3})
      df = df.astype(df_new_type)
      expected_rows = ["c", "mynull", "mynull", "mynull"]
      expected = tm.convert_rows_list_to_csv_str(expected_rows)
  
      result = df.to_csv(index=False, na_rep="mynull", encoding="ascii")
  
      assert expected == result
  
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 30 for calling the TestToCSV obj's underlying member method code with example usage: obj.test_to_csv_na_rep_long_string(df_new_type) without return types