code: def test_str():
      pair = TradingPair(USD, BTC)
      assert str(pair) == "USD/BTC"
  
imports: import pytest
  from tensortrade.oms.instruments import TradingPair, BTC, USD
  from tensortrade.core.exceptions import InvalidTradingPair
label_desc: Use image node_id 3 for calling a global function with example usage: test_str() without return types