7.4 一个经典的参数错误

a, b 都是整型时

def add(a, b):
    a += b
    return a

if __name__ == '__main__':
    a, b = 1, 2
    c = add(a, b)
    print('a: %s, b: %s, c: %s' % (a, b, c))
    # 结果为 a: 1, b: 2, c: 3
    # a 未发生变化

a, b 都是列表时

def add(a, b):
    a += b
    return a

if __name__ == '__main__':
    a, b = [1, 2], [3, 4]
    c = add(a, b)
    print('a: %s, b: %s, c: %s' % (a, b, c))
    # 结果为 a: [1, 2, 3, 4], b: [3, 4], c: [1, 2, 3, 4]
    # a 发生变化

a, b 都是元组时

def add(a, b):
    a += b
    return a


if __name__ == '__main__':
    a, b = (1, 2), (3, 4)
    c = add(a, b)
    print('a: %s, b: %s, c: %s' % (a, b, c))
    # 结果为 a: (1, 2), b: (3, 4), c: (1, 2, 3, 4)
    # a 未发生变化

默认类型为可变类型时

class Company:
    def __init__(self, name, staff_list=[]):
        self.name = name
        self.staff_list = staff_list

    def add(self, staff):
        self.staff_list.append(staff)

    def remove(self, staff):
        self.staff_list.remove(staff)


if __name__ == '__main__':
    com1 = Company('com1', ['staff1', 'staff11'])
    com1.add('staff111')
    com1.remove('staff11')
    print(com1.staff_list)

    com2 = Company('com2')
    com2.add('staff2')

    com3 = Company('com3')
    com3.add('staff3')

    print(com2.staff_list)  # ['staff2', 'staff3']
    print(com3.staff_list)  # ['staff2', 'staff3']

• 默认值为可变类型 []，com2.staff_list 和 com3.staff_list 指向一块内存空间