python的形参为可变数据类型的坑

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函数可能会修改接收到的任何可变对象。不要使用可变类型的参数作为函数的参数默认值。

class Bus:
    def __init__(self, passengers=[]):
        self.passengers = passengers

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

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


bus1 = Bus(['a', 'b'])
print(bus1.passengers)
bus1.pick('c')
bus1.drop('a')
print(bus1.passengers)

bus2 = Bus()
bus2.pick('a')
print(bus2.passengers)

bus3 = Bus()
bus3.pick('b')
print(bus3.passengers)
print(bus2.passengers)



['a', 'b']
['b', 'c']
['a']
['a', 'b']
['a', 'b']

# 浅复制的实际用处:保障篮球队的名单不变
basketball_team = ['a', 'b', 'c']


class TwilightBus:
    def __init__(self, passengers=None):
        if passengers is None:
            self.passengers = []
        else:
            self.passengers = list(passengers)

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


bus1 = TwilightBus(basketball_team)

bus1.drop('a')
bus1.drop('b')

print(basketball_team)#['a', 'b', 'c']

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