'Calculates the commission for an order execution.
:param order: The order being executed.
:type order: :class:`pyalgotrade.broker.Order`.
:param price: The price for each share.
:type price: float.
:param quantity: The order size.
:type quantity: float.
:rtype: float.'

@abc.abstractmethod
def calculate(self, order, price, quantity):

    raise NotImplementedError()
