code: def test_asfreq_fillvalue(self):
      # test for fill value during upsampling, related to issue 3715
  
      # setup
      rng = date_range("1/1/2016", periods=10, freq="2s")
      # Explicit cast to 'float' to avoid implicit cast when setting None
      ts = Series(np.arange(len(rng)), index=rng, dtype="float")
      df = DataFrame({"one": ts})
  
      # insert pre-existing missing value
      df.loc["2016-01-01 00:00:08", "one"] = None
  
      actual_df = df.asfreq(freq="1s", fill_value=9.0)
      expected_df = df.asfreq(freq="1s").fillna(9.0)
      expected_df.loc["2016-01-01 00:00:08", "one"] = None
      tm.assert_frame_equal(expected_df, actual_df)
  
      expected_series = ts.asfreq(freq="1s").fillna(9.0)
      actual_series = ts.asfreq(freq="1s", fill_value=9.0)
      tm.assert_series_equal(expected_series, actual_series)
  
imports: from datetime import datetime
  import numpy
  import pytest
  from pandas._libs.tslibs.offsets import MonthEnd
  from pandas import DataFrame, DatetimeIndex, Series, date_range, period_range, to_datetime
  import pandas._testing
  from pandas.tseries import offsets
label_desc: Use image node_id 11 for calling the TestAsFreq obj's underlying member method code with example usage: obj.test_asfreq_fillvalue() without return types