`class SpreadData:

def calculate_price(self):
    """"""
    self.clear_price()

    # Go through all legs to calculate price
    for n, leg in enumerate(self.legs.values()):   
        # Filter not all leg price data has been received
        if not leg.bid_volume or not leg.ask_volume:  
            self.clear_price()
            return

        # Calculate price
        price_multiplier = self.price_multipliers[leg.vt_symbol]   
        if price_multiplier > 0:
            self.bid_price += leg.bid_price * price_multiplier      
            self.ask_price += leg.ask_price * price_multiplier
        else:
            self.bid_price += leg.ask_price * price_multiplier        
            self.ask_price += leg.bid_price * price_multiplier`

这个是价差交易里面计算价格的函数
self.bid_price += leg.bid_price price_multiplier 这一行一直没有弄明白，为什么价差的买价只用单条腿的买价价格乘数计算？
而不是用主动腿（leg1)和被动腿(leg2)的差值计算？self.bid_price += leg1.bid_price price_multiplier - leg2.ask_price price_multiplier?
请教各位。

在.vntrader同级目录下创建了strategies文件夹，启动VN Trader的时候，加载不了策略，同时运行vscode的时候，提示 no module named ‘vnpy’，请问怎么解决这个问题