code: def _test_load_properly_forward_fills(self):
      engine = SimplePipelineEngine(
          lambda x: self.loader,
          self.trading_days,
          self.asset_finder,
      )
  
      # Cut the dates in half so we need to forward fill some data which
      # is not in our window. The results should be computed the same as if
      # we had computed across the entire window and then sliced after the
      # computation.
      dates = self.trading_days[len(self.trading_days) / 2 :]
      results = engine.run_pipeline(
          Pipeline({c.name: c.latest for c in EventDataSet.columns}),
          start_date=dates[0],
          end_date=dates[-1],
      )
  
      for c in EventDataSet.columns:
          if c in self.next_value_columns:
              self.check_next_value_results(
                  c,
                  results[c.name].unstack(),
                  dates,
              )
          elif c in self.previous_value_columns:
              self.check_previous_value_results(
                  c,
                  results[c.name].unstack(),
                  dates,
              )
          else:
              raise AssertionError("Unexpected column %s." % c)
  
imports: from datetime import time
  import itertools
  from itertools import product
  import blaze
  from nose_parameterized import parameterized
  import numpy
  import pandas
  from catalyst.pipeline import Pipeline, SimplePipelineEngine
  from catalyst.pipeline.common import EVENT_DATE_FIELD_NAME, TS_FIELD_NAME, SID_FIELD_NAME
  from catalyst.pipeline.data import DataSet, Column
  from catalyst.pipeline.loaders.events import EventsLoader
  from catalyst.pipeline.loaders.blaze.events import BlazeEventsLoader
  from catalyst.pipeline.loaders.utils import next_event_indexer, normalize_timestamp_to_query_time, previous_event_indexer
  from catalyst.testing import check_arrays, CatalystTestCase
  from catalyst.testing.fixtures import WithAssetFinder, WithTradingSessions
  from catalyst.testing.predicates import assert_equal
  from catalyst.utils.numpy_utils import categorical_dtype, datetime64ns_dtype, float64_dtype, int64_dtype
label_desc: Use image node_id 4 for calling the EventsLoaderTestCase obj's underlying member method code with example usage: obj._test_load_properly_forward_fills() without return types