code: def test_str(mock_exchange):
      exchange = mock_exchange.return_value
      exchange.name = "bitfinex"
  
      exchange_pair = ExchangePair(exchange, USD / BTC)
  
      assert str(exchange_pair) == "bitfinex:USD/BTC"
  
imports: import unittest.mock
  from tensortrade.oms.instruments import ExchangePair, BTC, USD
label_desc: Use image node_id 2 for calling a global function with example usage: test_str(mock_exchange) without return types