code: def _get_first_trading_day(self, assets):
      first_date = None
      for asset in assets:
          if first_date is None or asset.start_date > first_date:
              first_date = asset.start_date
      return first_date
  
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 DataPortalExchangeBacktest obj's underlying member method code with example usage: obj._get_first_trading_day(assets) and returns: first_date