按使用文档回测时获取主连合约标的，失败

我按文档里的代码，直接复制黏贴（账户密码改了），获取主连合约标的，但并没有按文档里的预期输出，只在第1个print下面有输出，具体如图2，请问为什么

from datetime import date
from tqsdk import TqApi, TqAuth, TqBacktest, BacktestFinished
 api = TqApi(backtest=TqBacktest(start_dt=date(2020, 1, 1), end_dt=date(2020, 10, 1)), auth=TqAuth("快期账户", "账户密码"))
 quote = api.get_quote("KQ.m@CFFEX.T")
print(quote.datetime, quote.underlying_symbol)
try:
    while True:
        api.wait_update()
        if api.is_changing(quote, "underlying_symbol"):
            print(quote.datetime, quote.underlying_symbol)
except BacktestFinished:
    api.close()
 # 预期输出：
# 2019-12-31 15:14:59.999999 CFFEX.T2003
# 2020-02-19 09:15:00.000000 CFFEX.T2006
# 2020-05-14 09:15:00.000000 CFFEX.T2009
# 2020-08-19 09:30:00.000000 CFFEX.T2012