code: def test_to_csv_write_to_open_file(self):
      # GH 21696
      df = DataFrame({"a": ["x", "y", "z"]})
      expected = """\
  manual header
  x
  y
  z
  """
      with tm.ensure_clean("test.txt") as path:
          with open(path, "w", encoding="utf-8") as f:
              f.write("manual header\n")
              df.to_csv(f, header=None, index=None)
          with open(path, encoding="utf-8") as f:
              assert f.read() == expected
  
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 23 for calling the TestToCSV obj's underlying member method code with example usage: obj.test_to_csv_write_to_open_file() without return types