再请教一下下面这段代码为什么获取不到数据。
from vnpy.trader.object import HistoryRequest
from vnpy.trader.rqdata import rqdata_client
from vnpy.trader.database import database_manager
from vnpy.trader.utility import extract_vt_symbol
from datetime import datetime
symbol, exchange = extract_vt_symbol("IF88.CFFEX")
req = HistoryRequest(
symbol=symbol,
exchange=exchange,
interval='1m',
start=datetime(2019, 1, 1),
end=datetime(2019, 4, 30)
)
result = rqdata_client.init()
print(result)
data = rqdata_client.query_history(req)
print(data)