Python父子类继承：子类如何继承父类属性并解决属性缺失报错

现有定义用户档案的Profile父类，包含account_holder、account_type、value属性及info方法。创建继承自Profile的Business子类时重写了__init__方法，添加自身属性与方法，但调用子类实例的info方法时出现AttributeError，提示不存在account_holder属性。请问如何在创建子类实例时传入父类的account_holder、account_type、value属性？或者是否应为这些属性设置默认值？

父类代码 #

class Profile: 

    # this is the class constructor 
    def __init__(self, account_holder, account_type, value): 
        self.account_holder = account_holder
        self.account_type = account_type
        self.value = value

    # some methods for the user account: 
    def info(self): 
        print(self.account_holder)
        print(self.account_type)
        print(self.value)


# create object: 

personal_acc = Profile(account_holder='Me', 
account_type='main account', 
value= 100)

personal_acc.info()

子类代码 #

class Business(Profile): 

    # this is a child class: 
    def __init__(self, business_name, business_value = 100): 
        self.business_name = business_name
        self.business_value = business_value

    # new_method

    def increase_value(self, increase = 400):
        self.business_value += increase


meta = Business(business_name='whatsapp', business_value = 1000)

meta.increase_value()

print(meta.business_value)

# print a method of the parent class using child class Business: 

meta.info()

报错信息 #

Traceback (most recent call last):
File "c:\Users\beans\test.py", line 558, in
meta.info()
File "c:\Users\beans\test.py", line 523, in info
print(self.account_holder)
AttributeError: 'Business' object has no attribute 'account_holder'

错误原因 #

重写子类__init__方法时，没有调用父类的__init__构造方法，导致父类定义的account_holder、account_type、value属性从未被初始化，调用父类info方法时自然找不到这些属性。

解决方案 #

方案1：显式调用父类构造方法，强制传入父类参数 #

修改子类__init__，先通过super()调用父类构造方法，完成父类属性的初始化，再处理子类自身属性：

class Business(Profile): 
    def __init__(self, account_holder, account_type, value, business_name, business_value=100): 
        # 调用父类构造方法，传入父类所需参数
        super().__init__(account_holder, account_type, value)
        self.business_name = business_name
        self.business_value = business_value

    def increase_value(self, increase=400):
        self.business_value += increase

# 创建实例时必须传入父类和子类的所有参数
meta = Business(account_holder='Meta Inc.', account_type='business account', value=50000, business_name='whatsapp', business_value=1000)
meta.info()  # 可正常输出父类属性

方案2：为父类参数设置默认值（可选） #

如果业务允许父类属性有默认值，可以修改父类__init__的参数默认值，这样子类实例化时可灵活选择是否传入父类参数：

修改父类：

class Profile: 
    def __init__(self, account_holder='Unknown', account_type='default', value=0): 
        self.account_holder = account_holder
        self.account_type = account_type
        self.value = value

    def info(self): 
        print(self.account_holder)
        print(self.account_type)
        print(self.value)

子类实现：

class Business(Profile): 
    def __init__(self, business_name, business_value=100, account_holder='Unknown', account_type='business', value=0): 
        super().__init__(account_holder, account_type, value)
        self.business_name = business_name
        self.business_value = business_value

# 仅传入子类参数，使用父类默认属性值
meta = Business(business_name='whatsapp', business_value=1000)
meta.info()  # 输出：Unknown、business、0

# 也可以主动传入父类参数
meta2 = Business(account_holder='Meta Inc.', account_type='corporate', value=100000, business_name='facebook', business_value=5000)
meta2.info()  # 输出：Meta Inc.、corporate、100000

方案选择 #

• 若父类属性是业务必需项，优先用方案1，确保实例化时必须传入完整参数；
• 若父类属性允许缺省，用方案2可简化实例化流程。

内容的提问来源于stack exchange，提问作者Beans On Toast