AttributeError Traceback (most recent call last)
<ipython-input-11-cda7b0c3a5b8> in <module>
     47         low_price=df.low[i],
     48         close_price=df.close[i],
---> 49         gateway_name="DB",
     50         )
     51     bars.append(bar)
<string> in init(self, gateway_name, symbol, exchange, datetime, interval, volume, open_interest, open_price, high_price, low_price, close_price)
c:\vnstudio\lib\site-packages\vnpy\trader\object.py in post_init(self)
     97     def post_init(self):
     98         """"""
---> 99         self.vt_symbol = f"{self.symbol}.{self.exchange.value}"
    100
    101
AttributeError: 'str' object has no attribute 'value'