防御可变参数

class Bus:
    def __init__(self,passengers = None):
        if passengers is None:
            self.passengers = []
        else:
            self.passengers = list(passengers)#创建副本,防御可变参数

    def pick(self,name):
        self.passengers.append(name)

    def drop(self,name):
        self.passengers.remove(name)