code: def test_fillna_bug(self):
      ser = Series(
          [np.nan, 1.0, np.nan, 3.0, np.nan], ["z", "a", "b", "c", "d"]
      )
      filled = ser.fillna(method="ffill")
      expected = Series([np.nan, 1.0, 1.0, 3.0, 3.0], ser.index)
      tm.assert_series_equal(filled, expected)
  
      filled = ser.fillna(method="bfill")
      expected = Series([1.0, 1.0, 3.0, 3.0, np.nan], ser.index)
      tm.assert_series_equal(filled, expected)
  
imports: from datetime import datetime, timedelta, timezone
  import numpy
  import pytest
  import pytz
  from pandas import Categorical, DataFrame, DatetimeIndex, NaT, Period, Series, Timedelta, Timestamp, date_range, isna, timedelta_range
  import pandas._testing
  from pandas.core.arrays import period_array
label_desc: Use image node_id 1 for calling the TestFillnaPad obj's underlying member method code with example usage: obj.test_fillna_bug() without return types