code: def test_price_ds():
      btc_price = Stream.source(
          [7000, 7500, 8300], dtype="float"
      ).rename("USD-BTC")
      eth_price = Stream.source([200, 212, 400], dtype="float").rename(
          "USD-ETH"
      )
  
      feed = DataFeed([btc_price, eth_price])
  
      assert feed.next() == {"USD-BTC": 7000, "USD-ETH": 200}
  
file_path: tensortrade/tests/tensortrade/unit/env/default/test_observers.py
imports: from tensortrade.env.default.observers import _create_internal_streams, _create_wallet_source
  from tensortrade.feed.core import DataFeed, Stream
  from tensortrade.oms.exchanges import Exchange
  from tensortrade.oms.services.execution.simulated import execute_order
  from tensortrade.oms.wallets import Wallet, Portfolio
  from tensortrade.oms.instruments import USD, BTC, ETH, LTC, Quantity
label_desc: Use image node_id 1 for calling a global function with example usage: test_price_ds() without return types