如何从具有__new__方法的超类中正确继承？ [英] how do I properly inherit from a superclass that has a __new__ method?

问题描述

假设我们有一个类'Parent'，由于某种原因定义了 __ new __ ，并且继承了它的类'Child'。
（在我的情况下，我试图继承我无法修改的第三方课程）

Let's assume we have a class 'Parent' , that for some reason has __new__ defined and a class 'Child' that inherits from it. (In my case I'm trying to inherit from a 3rd party class that I cannot modify)

class Parent:
    def __new__(cls, arg):
        # ... something important is done here with arg

我的尝试是：

class Child(Parent):
    def __init__(self, myArg, argForSuperclass):
         Parent.__new__(argForSuperclass)
         self.field = myArg

但是

p = Parent("argForSuperclass")

按预期工作

c = Child("myArg", "argForSuperclass")

失败，因为'Child'试图调用 __new __ 方法它继承自'Parent'而不是它自己的 __ init __ 方法。

fails, because 'Child' tries to call the __new__ method it inherits from 'Parent' instead of its own __init__ method.

为了获得预期的行为，我需要在'Child'中进行哪些更改？

What do I have to change in 'Child' to get the expected behavior?

推荐答案

F首先，完全覆盖 __ new __ 并不是最佳做法，以避免这些问题......但我知道，这不是你的错。对于这种情况，覆盖 __ new __ 的最佳做法是使其接受可选参数...

Firstly, it is not considered best practice to override __new__ exactly to avoid these problems... But it is not your fault, I know. For such cases, the best practice on overriding __new__ is to make it accept optional parameters...

class Parent(object):
    def __new__(cls, value, *args, **kwargs):
        print 'my value is', value
        return object.__new__(cls, *args, **kwargs)

...所以孩子们可以收到自己的：

...so children can receive their own:

class Child(Parent):
    def __init__(self, for_parent, my_stuff):
        self.my_stuff = my_stuff

然后，它会起作用：

>>> c = Child(2, "Child name is Juju")
my value is 2
>>> c.my_stuff
'Child name is Juju'

但是，你父类的作者这不是那么明智并给你这个问题：

However, the author of your parent class was not that sensible and gave you this problem:

class Parent(object):
    def __new__(cls, value):
        print 'my value is', value
        return object.__new__(cls)

在这种情况下，只需覆盖子项中的 __ new __ ，使其接受可选参数，并调用父项的 __ new __ 那里：

In this case, just override __new__ in the child, making it accept optional parameters, and call the parent's __new__ there:

class Child(Parent):
    def __new__(cls, value, *args, **kwargs):
        return Parent.__new__(cls, value)
    def __init__(self, for_parent, my_stuff):
        self.my_stuff = my_stuff

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