Python:从子类更改父类属性

Question

在 python (v3.6.1) 中，我希望编写一个这样的类:

class SecondClass(FirstClass):
    property = "custom"

print(SecondClass.name) #=> "john"

class SecondClass(FirstClass):
    property = "notcustom"

print(SecondClass.name) #=> "steve"

我尝试像这样设置 FirstClass 类:

class FirstClass:
    if property == "custom":
        name = "john"
    else:
        name = "steve"

但我似乎无法从 SecondClass 编辑 FirstClass 的属性。

这可能吗？

Answer 1

由于您使用的是 Python 3.6，因此您可以使用新的 __init_subclass__ 来完成您的要求。方法。来自 __init_subclass__ 的文档:

This method is called whenever the containing class is subclassed. cls is then the new subclass. If defined as a normal instance method, this method is implicitly converted to a class method.

class FirstClass:
    def __init_subclass__(cls):
        super().__init_subclass__()
        if cls.property == "custom":
            cls.name = "john"
        else:
            cls.name = "steve"

class SecondClass(FirstClass):
    property = "custom"

print(SecondClass.name)

class SecondClass(FirstClass):
    property = "notcustom"

print(SecondClass.name) 

对于适用于 Python 3.5 及更低版本的方法，您可以使用一些元类魔法:

class FirstClass(type):
    def __init__(cls, name, bases, attrs):
        if cls.property == "custom":
            cls.name = "john"
        else:
            cls.name = "steve"
        super(FirstClass, cls).__init__(name, bases, attrs)

class SecondClass(metaclass=FirstClass):
    property = "custom"

print(SecondClass.name)

class SecondClass(metaclass=FirstClass):
    property = "notcustom"

print(SecondClass.name)