简单的python继承 [英] Simple python inheritance

问题描述

class Animal(object):
    def __init__(self, nlegs=4):
        print '__init__ Animal'
        self.nlegs = nlegs

class Cat(Animal):
    def __init__(self, talk='meow'):
        print '__init__ Cat'
        self.talk = talk

class Dog(Animal):
    def __init__(self, talk='woof'):
        print '__init__ Dog'
        self.talk = talk

1. 为什么我的猫 tom = Cat（）没有nlegs属性？


2. 我们是否应该从 Cat .__ init __ Animal .__ init __（） >，或者我们应该用 super 做一些更有趣的事情？


3. 如果我想创建一条有5条腿的猫，我需要在 Cat .__ init __ 界面中添加其他参数吗？ ？

1. Why does my cat tom = Cat() not have an nlegs attribute?
2. Should we explicitly call Animal.__init__() from within Cat.__init__, or should we be doing something more fancy, say, with super?
3. What about if I want to create a cat with 5 legs, do I need to add additional arguments to the Cat.__init__ interface?

推荐答案

建立在其他人所说的基础之上，是的，你需要调用父的 __ init __ 方法。

To build on what everyone else has said, yes, you'll need to call the parent's __init__ method.

通常最好使用super。但是，在某些情况下（特别是当你从多个类继承时）它可能是一个很大的问题。我将避免详细介绍，不缺少讨论它的各种文章。 （另外，其他一些特殊功能也有些奇怪。例如，你可以做 super（SomeCls，self）.__ getitem __（5）但是 super（SomeCls，self）[5] 将不起作用。）

It's generally best to use super. However, in certain cases (particularly when you're inheriting from multiple classes) it can be a big gotcha. I'll avoid going into detail, there are no shortage of various articles which discuss it. (Also, there are some oddities with some of the other "special" functions. For example, you can do super(SomeCls, self).__getitem__(5) but super(SomeCls, self)[5] won't work.)

作为一个简单的例子说明为什么它是好的想要使用它，你可以让狗和猫继承自哺乳动物（继承自 Animal ）并且不必更改代码中除 Dog 之外的其他地方 Cat 继承自。

As a simplistic example of why it's a good idea to use it, you could make Dog and Cat inherit from Mammal (which inherits from Animal) and not have to change places in your code other than which class Dog and Cat inherit from.

至于为什么你的 tom 实例没有 tom.nlegs ，这是因为你没有调用 Animal 的 __ init __ 方法。

As for why your tom instance doesn't have tom.nlegs, it's because you haven't called Animal's __init__ method.

还要记住，并非所有内容都需要在初始化时设置。对于此示例，更有意义的是不在 __ init __ 方法中设置 nlegs 之类的内容。相反，只需在类中直接设置它。例如

Also remember that not everything needs to be set at initialization time. For this example, it makes more sense not to set things like nlegs in the __init__ method. Instead, just set it directly in the class. E.g.

class Mammal(object):
    nlimbs = 4
    def __init__(self):
        print "I'm a mammal!"

class Cat(Mammal):
    def __init__(self, color="calico"):
        self.color = color
        super(Cat, self).__init__()
        print "I have {0} legs I am {1}".format(self.nlimbs, self.color)

class FiveLeggedCat(Cat):
    nlimbs = 5

基本上，如果某些事情可能会在不同实例之间发生变化（例如cat）或者需要在初始化时完成（例如打开文件），然后它可能应该在 __ init __ 中设置。

Basically, if something is likely to change from instance to instance (e.g. the color of the cat) or needs to be done at initialization (e.g. opening a file), then it probably should be set in __init__.

否则，如果我们想要对类的任何实例都相同，那么直接在类定义中设置它可能更清晰。

Otherwise, if it's something we want to be the same for any instance of the class, it can be cleaner to set it directly in the class definition.

此外，以这种方式设置的属性将可用于文档工具（例如内置的 help 函数），而在初始化时设置的属性不会。

Also, attributes set this way will be available to documentation tools (e.g. the built-in help function), whereas attributes set at initialization won't be.

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