code: def group_assets_by_exchange(assets):
      exchange_assets = dict()
      for asset in assets:
          if asset.exchange not in exchange_assets:
              exchange_assets[asset.exchange] = list()
  
          exchange_assets[asset.exchange].append(asset)
  
      return exchange_assets
  
imports: import hashlib
  import json
  import os
  import pickle
  import shutil
  from datetime import date, datetime
  import pandas
  from catalyst.assets._assets import TradingPair
  from six import string_types
  from six.moves.urllib import request
  from catalyst.constants import DATE_FORMAT, SYMBOLS_URL
  from catalyst.exchange.exchange_errors import ExchangeSymbolsNotFound
  from catalyst.exchange.utils.serialization_utils import ExchangeJSONEncoder, ExchangeJSONDecoder
  from catalyst.utils.memoize import weak_lru_cache
  from catalyst.utils.paths import data_root, ensure_directory, last_modified_time
label_desc: Use image node_id 28 for calling a global function with example usage: group_assets_by_exchange(assets) and returns: exchange_assets