code: def fill_listener():
      class FillListener(OrderListener):
          def __init__(self):
              self.listened = False
  
          def on_fill(self, order: "Order", trade: "Trade"):
              self.listened = True
  
      return FillListener()
  
file_path: tensortrade/tests/tensortrade/unit/oms/orders/test_order_listener.py
imports: import pytest
  import unittest.mock
  from decimal import Decimal
  from tensortrade.oms.orders import OrderListener, Order, TradeType, TradeSide
  from tensortrade.oms.instruments import USD, BTC, ExchangePair
  from tensortrade.oms.wallets import Wallet, Portfolio
  from tensortrade.oms.exchanges import ExchangeOptions
label_desc: Use image node_id 2 for calling a global function with example usage: fill_listener() and returns: FillListener