我应该在抽象方法的主体中放什么? [英] What should I put in the body of an abstract method?

问题描述

假设我有以下抽象类 Foo:

Say I have the following abstract class Foo:

import abc

class Foo(abc.ABC):

    @abc.abstractmethod
    def bar(self):
        raise NotImplementedError

我应该在 bar 方法的主体中放什么?

What should I put in the body of the bar method?

我看到很多代码都raise NotImplementedError，如上所示.但是，这似乎是多余的，因为任何未实现 bar 的子类都会在实例化时引发 TypeError: Can't instanceiate abstract class Foo with abstract methods bar.

I see a lot of code that has raise NotImplementedError, as shown above. However, this seems redundant, since any subclass that does not implement bar will raise the TypeError: Can't instantiate abstract class Foo with abstract methods bar when it is instantiated.

把bar留空是不是Pythonic，如下:

Is it Pythonic to leave bar empty, as follows:

import abc

class Foo(abc.ABC):

    @abc.abstractmethod
    def bar(self):
        ...

这是在 Python 文档中为 抽象基类，但我不确定这只是一个占位符还是如何编写代码的实际示例.

This is what is done in the Python docs for Abstract Base Classes, but I'm not sure if that's just a placeholder or an actual example of how to write code.

如果让 bar 只留下三个点 (...) 就可以了，我什么时候应该使用 NotImplementedError?

If it's ok to leave bar with only three dots (...), when should I use NotImplementedError?

推荐答案

该文档旨在为您提供一个示例.您不必遵循它.

The documentation does aim to give you an example. You don't have to follow it.

你可以提供一个默认值；子类仍然可以自由地使用 super() 来调用您的实现.这是大多数 collections.abc 类所做的；请参阅源代码.

You could provide a default; subclasses are still free to use super() to call your implementation. This is what most of the collections.abc classes do; see the source code.

Size 例如，对于__len__返回0:

class Sized(metaclass=ABCMeta):
    # ...
    @abstractmethod
    def __len__(self):
        return 0

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