code: def _get_history_window(
      self,
      assets,
      end_dt,
      bar_count,
      frequency,
      field,
      data_frequency,
      ffill=True,
  ):
      exchange_assets = group_assets_by_exchange(assets)
      if len(exchange_assets) > 1:
          df_list = []
          for exchange_name in exchange_assets:
              assets = exchange_assets[exchange_name]
  
              df_exchange = self.get_exchange_history_window(
                  exchange_name,
                  assets,
                  end_dt,
                  bar_count,
                  frequency,
                  field,
                  data_frequency,
                  ffill,
              )
  
              df_list.append(df_exchange)
  
          # Merging the values values of each exchange
          return pd.concat(df_list)
  
      else:
          exchange_name = list(exchange_assets.keys())[0]
          return self.get_exchange_history_window(
              exchange_name,
              assets,
              end_dt,
              bar_count,
              frequency,
              field,
              data_frequency,
              ffill,
          )
  
imports: import abc
  import datetime
  import numpy
  import pandas
  from catalyst.assets._assets import TradingPair
  from catalyst.constants import LOG_LEVEL, AUTO_INGEST
  from catalyst.data.data_portal import DataPortal
  from catalyst.exchange.exchange_bundle import ExchangeBundle
  from catalyst.exchange.exchange_errors import ExchangeRequestError, PricingDataNotLoadedError
  from catalyst.exchange.utils.exchange_utils import resample_history_df, group_assets_by_exchange
  from catalyst.exchange.utils.datetime_utils import get_frequency, get_start_dt
  from logbook import Logger
  from redo import retry
label_desc: Use image node_id 2 for calling the DataPortalExchangeBase obj's underlying member method code with example usage: obj._get_history_window(assets, end_dt, bar_count, frequency, field, data_frequency, ffill) and returns: pd, self