code: def test_fillna_downcast_false(self, frame_or_series):
      # GH#45603 preserve object dtype with downcast=False
      obj = frame_or_series([1, 2, 3], dtype="object")
      msg = "The 'downcast' keyword in fillna"
      with tm.assert_produces_warning(FutureWarning, match=msg):
          result = obj.fillna("", downcast=False)
      tm.assert_equal(result, obj)
  
imports: import numpy
  import pytest
  from pandas._config import using_pyarrow_string_dtype
  from pandas import Categorical, DataFrame, DatetimeIndex, NaT, PeriodIndex, Series, TimedeltaIndex, Timestamp, date_range, to_datetime
  import pandas._testing
  from pandas.tests.frame.common import _check_mixed_float
label_desc: Use image node_id 15 for calling the TestFillNA obj's underlying member method code with example usage: obj.test_fillna_downcast_false(frame_or_series) without return types