code: def test_pad_nan(self):
      x = Series(
          [np.nan, 1.0, np.nan, 3.0, np.nan],
          ["z", "a", "b", "c", "d"],
          dtype=float,
      )
  
      return_value = x.fillna(method="pad", inplace=True)
      assert return_value is None
  
      expected = Series(
          [np.nan, 1.0, 1.0, 3.0, 3.0],
          ["z", "a", "b", "c", "d"],
          dtype=float,
      )
      tm.assert_series_equal(x[1:], expected[1:])
      assert np.isnan(x.iloc[0]), np.isnan(expected.iloc[0])
  
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 5 for calling the TestFillnaPad obj's underlying member method code with example usage: obj.test_pad_nan() without return types