code: def fillna():
      NUM_COLS = 3
      df = h2o.create_frame(
          rows=1000000,
          cols=NUM_COLS,
          real_fraction=1.0,
          real_range=100,
          missing_fraction=0.2,
          seed=123,
      )
      # Pandas comparison
      pdf = df.as_data_frame()
      filledpdf = pdf.fillna(method="ffill", axis=0, limit=3)
      filledpdfh2o = h2o.H2OFrame(
          filledpdf, column_types=["float"] * NUM_COLS
      )
      filled = df.fillna(method="forward", axis=0, maxlen=3)
      assert (
          abs((filled - filledpdfh2o).sum(return_frame=False)) < 1e-11
      ), "Difference between Pandas pivot too high"
  
imports: import sys
  import h2o
  import pandas
  from tests import pyunit_utils
label_desc: Use image node_id 1 for calling a global function with example usage: fillna() without return types