code: def test_to_csv_with_mix_columns(self):
      # gh-11637: incorrect output when a mix of integer and string column
      # names passed as columns parameter in to_csv
  
      df = DataFrame({0: ["a", "b", "c"], 1: ["aa", "bb", "cc"]})
      df["test"] = "txt"
      assert df.to_csv() == df.to_csv(columns=[0, 1, "test"])
  
imports: import csv
  from io import StringIO
  import os
  import numpy
  import pytest
  from pandas.errors import ParserError
  import pandas
  from pandas import DataFrame, Index, MultiIndex, NaT, Series, Timestamp, date_range, period_range, read_csv, to_datetime
  import pandas._testing
  import pandas.core.common
  from pandas.io.common import get_handle
label_desc: Use image node_id 21 for calling the TestDataFrameToCSV obj's underlying member method code with example usage: obj.test_to_csv_with_mix_columns() without return types