code: def test_fillna(self):
      pdf = pd.DataFrame(
          {
              "A": [1, 1, 2, 2] * 3,
              "B": [2, 4, None, 3] * 3,
              "C": [None, None, None, 1] * 3,
              "D": [0, 1, 5, 4] * 3,
          }
      )
      pkey = pd.Series([1, 1, 2, 2] * 3)
      psdf = ps.from_pandas(pdf)
      kkey = ps.from_pandas(pkey)
  
      self.assert_eq(
          psdf.groupby(kkey).fillna(0).sort_index(),
          pdf.groupby(pkey).fillna(0).sort_index(),
      )
      self.assert_eq(
          psdf.groupby(kkey)["C"].fillna(0).sort_index(),
          pdf.groupby(pkey)["C"].fillna(0).sort_index(),
      )
      self.assert_eq(
          psdf.groupby(kkey)[["C"]].fillna(0).sort_index(),
          pdf.groupby(pkey)[["C"]].fillna(0).sort_index(),
      )
      self.assert_eq(
          psdf.groupby(kkey).fillna(method="bfill").sort_index(),
          pdf.groupby(pkey).fillna(method="bfill").sort_index(),
      )
      self.assert_eq(
          psdf.groupby(kkey)["C"].fillna(method="bfill").sort_index(),
          pdf.groupby(pkey)["C"].fillna(method="bfill").sort_index(),
      )
      self.assert_eq(
          psdf.groupby(kkey)[["C"]].fillna(method="bfill").sort_index(),
          pdf.groupby(pkey)[["C"]].fillna(method="bfill").sort_index(),
      )
      self.assert_eq(
          psdf.groupby(kkey).fillna(method="ffill").sort_index(),
          pdf.groupby(pkey).fillna(method="ffill").sort_index(),
      )
      self.assert_eq(
          psdf.groupby(kkey)["C"].fillna(method="ffill").sort_index(),
          pdf.groupby(pkey)["C"].fillna(method="ffill").sort_index(),
      )
      self.assert_eq(
          psdf.groupby(kkey)[["C"]].fillna(method="ffill").sort_index(),
          pdf.groupby(pkey)[["C"]].fillna(method="ffill").sort_index(),
      )
  
file_path: spark/python/pyspark/pandas/tests/diff_frames_ops/test_groupby_fillna.py
imports: import pandas
  from pyspark import pandas
  from pyspark.pandas.config import set_option, reset_option
  from pyspark.testing.pandasutils import PandasOnSparkTestCase
  from pyspark.testing.sqlutils import SQLTestUtils
label_desc: Use image node_id 3 for calling the GroupByFillNAMixin obj's underlying member method code with example usage: obj.test_fillna() without return types