code: def buy(self, security, price=0, amount=0, volume=0, entrust_prop=0):
          
          return self._trade(security, price, amount, volume, "buy")
docs: 买入卖出股票
          :param security: 股票代码
          :param price: 买入价格
          :param amount: 买入股数
          :param volume: 买入总金额 由 volume / price 取整， 若指定 price 则此参数无效
          :param entrust_prop: