Python有类原型（或前向声明）吗？ [英] Does Python have class prototypes (or forward declarations)?

问题描述

我在一个文件中有一系列Python类。

I have a series of Python classes in a file. Some classes reference others.

我的代码是这样的：

class A():
    pass

class B():
    c = C()

class C():
    pass

尝试运行，我得到 NameError：name'C'is not defined 。公平了，但有什么办法使它工作，或者我必须手动重新排序我的类适应？在C ++中，我可以创建一个类原型。 Python有一个等价的？

Trying to run that, I get NameError: name 'C' is not defined. Fair enough, but is there any way to make it work, or do I have to manually re-order my classes to accommodate? In C++, I can create a class prototype. Does Python have an equivalent?

（我实际上在使用Django模型，但我尝试不复杂的事情）。

(I'm actually playing with Django models, but I tried not complicate matters).

推荐答案

在Python中，你不会创建一个原型本身，但你需要了解类属性和实例级属性之间的区别。在上面的例子中，你在类B上声明一个类属性，而不是一个实例级属性。

In Python you don't create a prototype per se, but you do need to understand the difference between "class attributes" and instance-level attributes. In the example you've shown above, you are declaring a class attribute on class B, not an instance-level attribute.

这是你正在寻找的东西： / p>

This is what you are looking for:

class B():
    def __init__(self):
        self.c = C()

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