def calculate(self, transaction):
        commission = abs(transaction.amount * self.cost)
        if self.min_trade_cost is None:
            return self.cost, commission
        else:
            commission = max(commission, self.min_trade_cost)
            return abs(commission / transaction.amount), commission
returns a tuple of:
dataset/ETHPy150Open quantopian/zipline/zipline/finance/commission.py/PerShare.calculate