Python抽象property()“无法使用抽象方法实例化抽象类[]"，但我做到了 [英] Python abstract property() "Can't instantiate abstract class [] with abstract methods", but I did

问题描述

我正在尝试在python 3.7中创建具有多个抽象python属性的基类.

I'm trying to create a base class with a number of abstract python properties, in python 3.7.

我使用@ property，@ abstractmethod，@ property.setter批注尝试了一种方式(请参见下面的开始").这行得通，但如果子类未实现setter，它也不会引发异常.那就是对我使用@abstract的意义，所以这不好.

I tried it one way (see 'start' below) using the @property, @abstractmethod, @property.setter annotations. This worked but it doesn't raise an exception if the subclass doesn't implement a setter. That's the point of using @abstract to me, so that's no good.

所以我尝试使用两个@abstractmethod方法和一个'property()'来另一种方式(请参见下面的'end')，该方法本身不是抽象的，而是使用了那些方法.实例化子类时，这种方法会产生错误:

So I tried doing it another way (see 'end' below) using two @abstractmethod methods and a 'property()', which is not abstract itself but uses those methods. This approach generates an error when instantiating the subclass:

        # {TypeError}Can't instantiate abstract class FirstStep with abstract methods end

我显然正在实现抽象方法，所以我不明白它的含义.'end'属性未标记为@abstract，但是如果我将其注释掉，它确实可以运行(但是我没有得到我的属性).我还添加了测试非抽象方法"test_elapsed_time"，以证明我具有类结构和抽象权限(有效).

I'm clearly implementing the abstract methods, so I don't understand what it means. The 'end' property is not marked @abstract, but if I comment it out, it does run (but I don't get my property). I also added that test non-abstract method 'test_elapsed_time' to demonstrate I have the class structure and abstraction right (it works).

是否有可能我在做一些愚蠢的事情，或者在property()周围是否有一些特殊的行为导致这种情况?

Any chance I'm doing something dumb, or is there some special behavior around property() that's causing this?

class ParentTask(Task):
    def get_first_step(self):
        # {TypeError}Can't instantiate abstract class FirstStep with abstract methods end
        return FirstStep(self)


class Step(ABC):
    #     __metaclass__ = ABCMeta
    def __init__(self, task):
        self.task = task

    # First approach.  Works, but no warnings if don't implement setter in subclass
    @property
    @abstractmethod
    def start(self):
        pass

    @start.setter
    @abstractmethod
    def start(self, value):
        pass

    # Second approach.  "This method for 'end' may look slight messier, but raises errors if not implemented.
    @abstractmethod
    def get_end(self):
        pass

    @abstractmethod
    def set_end(self, value):
        pass

    end = property(get_end, set_end)

    def test_elapsed_time(self):
        return self.get_end() - self.start


class FirstStep(Step):
    @property
    def start(self):
        return self.task.start_dt

    # No warnings if this is commented out.
    @start.setter
    def start(self, value):
        self.task.start_dt = value

    def get_end(self):
        return self.task.end_dt

    def set_end(self, value):
        self.task.end_dt = value

推荐答案

我怀疑这是抽象方法和属性交互中的错误.

I suspect this is a bug in the interaction of abstract methods and properties.

在您的基类中，依次发生以下事情:

In your base class, the following things happen, in order:

1. 您定义了一个名为 start 的抽象方法.
2. 您创建一个新属性，该属性使用1)中的抽象方法作为其吸气剂.现在，名称 start 引用了此属性，并且仅引用了现在由 Self.start.fget 保存的原始名称.
3. Python保存对 start.setter 的临时引用，因为名称 start 即将绑定到另一个对象.
4. 您创建另一个名为 start
的抽象方法
5. 3)的引用被赋予4)的抽象方法，以定义一个新属性来替换当前绑定到名称 start 的属性.此属性具有从1开始的方法作为其getter，从4)开始作为其设置器.现在， start 引用了此属性； start.fget 是指1)中的方法； start.fset 是指4)中的方法.

1. You define an abstract method named start.
2. You create a new property that uses the abstract method from 1) as its getter. The name start now refers to this property, with the only reference to the original name now held by Self.start.fget.
3. Python saves a temporary reference to start.setter, because the name start is about to be bound to yet another object.
4. You create a second abstract method named start
5. The reference from 3) is given the abstract method from 4) to define a new property to replace the once currently bound to the name start. This property has as its getter the method from 1 and as its setter the method from 4). Now start refers to this property; start.fget refers to the method from 1); start.fset refers to the method from 4).

这时，您有了一个属性，其 component 函数是抽象方法.该属性本身没有被修饰为抽象的，但是 property .__ isabstractmethod __ 的定义将其标记为这样，因为其所有组成方法都是抽象的.更重要的是，您在 Step .__ abstractmethods __ 中具有以下条目:

At this point, you have a property, whose component functions are abstract methods. The property itself was not decorated as abstract, but the definition of property.__isabstractmethod__ marks it as such because all its component methods are abstract. More importantly, you have the following entries in Step.__abstractmethods__:

1. 开始，属性
2. end ，属性
3. set_end ， end
的设置器
4. gen_end ， end
的获取者

1. start, the property
2. end, the property
3. set_end, the setter for end
4. gen_end, the getter for end

请注意，缺少 start 属性的组件函数，因为 __ abstractmethods __ 存储了 的名称，而不是对需要引用的内容的引用被覆盖.使用 property 和结果属性的 setter 方法作为修饰符，可以反复替换 start 名称所指的内容.

Note that the component functions for the start property are missing, because __abstractmethods__ stores names of, not references to, things that need to be overriden. Using property and the resulting property's setter method as decorators repeatedly replace what the name start refers to.

现在，在子类中，您定义一个名为 start 的 new 属性，以隐藏继承的属性，该属性具有 no 设置器和一个具体方法作为其获取方法.此时，是否为该属性提供设置器都没有关系，因为就 abc 机制而言，您已提供了它所要求的一切:

Now, in your child class, you define a new property named start, shadowing the inherited property, which has no setter and a concrete method as its getter. At this point, it doesn't matter if you provide a setter for this property or not, because as far as the abc machinery is concerned, you have provided everything it asked for:

1. 名称 start
的具体方法
2. 名称为 get_end 和 set_end
的具体方法
3. 由于名称 end 的所有基础函数都已提供了具体定义，因此对名称 end 进行了具体定义.

1. A concrete method for the name start
2. Concrete methods for the names get_end and set_end
3. Implicitly a concrete definition for the name end, because all of the underlying functions for the property end have been provided concrete definitions.

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