继承和dunder方法？

Question

我目前正在学习OOP，并且遇到了Dunder方法和继承的问题。

我有两个父类(都有自己的初始化)，它们继承到一个子类，我想要一个在所有类中初始化的数据的字符串表示。

这就是问题：

#Parent class 1:

class Contact:

    all_contacts = []

    def __init__(self,name='',email='',**kwargs):
        self.name = name
        self.email = email
        Contact.all_contacts.append(self)

    def __str__(self):
        return f'the name is {self.name}, and the mail is {self.mail}.'

#Parent class 2:

class AddressHolder:
    def __init__(self, street='', city='', state='', code='', **kwargs):
        self.street = street
        self.city = city
        self.state = state
        self.code = code

    def __str__(self):
        return f'the street is {self.street}, the city is {self.city}, 
               the state is {self.state}, the code is {self.code}'

#Subclass that inherits from both parent classes:

class Friends(Contact, AddressHolder):
    def __init__(self, phone='', **kwargs):
        self.phone = phone
        super().__init__(**kwargs)

    def __str__(self):
        return f'''The phone is {self.phone}.'''

#The instance of the class

f = Friends(phone='aphone', name='aname', mail='amail', 
street='astreet', city='acity', state='astate', code='acode')

print(f)

这将仅打印电话初始化。

我试过这样做，这样所有的__str__都会被打印出来，但没有成功：

    def __str__(self):
        super(Contact.self).__str__()
        super(AddressHolder.self).__str__()
        return f'''The phone is {self.phone}.'''

最初，我尝试将所有内容都放到Friends子类中，因为它是继承的，它应该可以工作：

    def __str__(self):
        return f'''The phone is {self.phone},
                    the street is {self.street},
                    the city is {self.city},
                    the state is {self.state},
                    the code is {self.code},
                    the name is {self.name},
                    and the mail is {self.mail}.'''

但是我得到了一个街道错误：‘’对象没有属性‘AttributeError’‘错误(这不是我可以在子类上使用父类数据的继承点吗？)

我为每个类都提供了自己的__str__，这可以说是糟糕的代码，但也不起作用。

如何打印继承类加上子类的所有数据？

如果我得到一个“子类没有属性”的错误，继承的限制是什么？

Answer 1

你需要让你的基类协同工作，让它们都调用super().__init__()：

class Contact:

    all_contacts = []

    def __init__(self, name='', email='', **kwargs):
        super().__init__(**kwargs)
        self.name = name
        self.email = email
        Contact.all_contacts.append(self)

和

class AddressHolder:
    def __init__(self, street='', city='', state='', code='', **kwargs):
        super().__init__(**kwargs)
        self.street = street
        self.city = city
        self.state = state
        self.code = code

    def __str__(self):
        return f'the street is {self.street}, the city is {self.city}, 
               the state is {self.state}, the code is {self.code}'

这之所以有效，是因为super()从当前类(定义方法的类)开始，在type(self)方法解析顺序的基类中搜索命名属性(此处为__init__)。对于Friends实例，MRO为：

>>> Friends.__mro__
(<class '__main__.Friends'>, <class '__main__.Contact'>, <class '__main__.AddressHolder'>, <class 'object'>)

您已经使用关键字参数来传递额外的参数，因此在MRO的末尾将得到一个空的kwargs字典，有效地调用不带参数的object.__init__()。

请注意，您还可以在__str__方法中使用super()：

class Contact:
    # ...
    def __str__(self):
        return "\n".join([
            f'the name is {self.name}, and the mail is {self.mail}.',
            super().__str__()
        ])

class AddressHolder:
    # ...

    def __str__(self):
        return "\n".join([
            f'the street is {self.street}, the city is {self.city}, '
            f'the state is {self.state}, the code is {self.code}',
            super().__str__()
        ])

class Friends(Contact, AddressHolder):
    # ...

    def __str__(self):
        return "\n".join([
            f'The phone is {self.phone}.',
            super().__str__()

这适用于object定义的任何属性：

>>> print(*dir(object()), sep="\n")
__class__
__delattr__
__dir__
__doc__
__eq__
__format__
__ge__
__getattribute__
__gt__
__hash__
__init__
__init_subclass__
__le__
__lt__
__ne__
__new__
__reduce__
__reduce_ex__
__repr__
__setattr__
__sizeof__
__str__
__subclasshook__

Answer 2

您可以尝试这样做：

class A:
    def __init__(self, a):
        self.a = a

    def __str__(self):
        return self.a

class B:
    def __init__(self, b):
        self.b = b

    def __str__(self):
        return self.b

class C(A, B):
    def __init__(self, a, b, c):
        A.__init__(self, a)
        B.__init__(self, b)
        self.c = c

    def __str__(self):
        return A.__str__(self) + B.__str__(self) + self.c

c = C("a", "b", "c")
print(c)

这将打印出abc。只需将a、b和c替换为所需的值即可。