什么是属性对象? [英] What is a property object?

问题描述

我对 Python 并不陌生，但我在这里有一个非常基本的问题.

我在玩python，发现有type属性

<预><代码>>>>财产<输入属性">

但我只听说过函数上下文中的属性.

<预><代码>>>>a = 属性()<0x0246C090处的属性对象>

但是属性对象呢?他们有什么用?属性方法不是很直观或暗示性

<预><代码>>>>目录(一)['__class__', '__delattr__', '__delete__', '__doc__', '__format__', '__get__', '__getattribute__', '__hash__', '__init__', '__new__', '__reduce__', '__reduce'__ex__'__repr__', '__set__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', 'deleter', 'fdel', 'fget', 'fset', 'getter', 'setter']

感谢您的关注！

解决方案

property 对象实际上是您认为的属性.考虑这个例子:

class Foo(object):def __init__(self):self._bar = 0@财产定义栏(自我):返回 self._bar + 5

Foo.bar 是一个具有 __get__ 方法的属性对象.当你写一些像

x = Foo()打印(x.bar)

查找 x.bar 发现 type(x).bar 有一个 __get__ 方法，因此属性查找变得等效

type(x).bar.__get__(x, type(x))

产生值 x._bar + 5.

使用 property 作为装饰器在某种程度上掩盖了 bar 是一个 property 对象的事实.一个等效的定义是

class Foo(object):def __init__(self):self._bar = 0bar = 属性(lambda 自我:self._bar + 5)

它更明确地表明您正在创建一个 property 对象，使用给定的 lambda 表达式作为该属性的 getter，并将该对象绑定到类属性 bar.

property 类(以及实例方法、类方法和静态方法)是 Python 通用描述符协议的具体应用，它通过 __get__、__set__ 和/或 __del__ 方法.

I am not new to python, but I have a pretty basic question here.

I was playing around with python and found that there is the type property

>>> property
<type 'property'>

But I have only heard of properties in the function context.

>>> a = property()
<property object at 0x0246C090>

But what about property objects? What are they use? Property methods are not very intuitive or suggestive

>>> dir(a)
['__class__', '__delattr__', '__delete__', '__doc__', '__format__', '__get__', '__getattribute__', '__hash__', '__init__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__set__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', 'deleter', 'fdel', 'fget', 'fset', 'getter', 'setter']

Thank you for the attention!

解决方案

The property object is what you are actually thinking of as a property. Consider this example:

class Foo(object):
    def __init__(self):
        self._bar = 0

    @property
    def bar(self):
        return self._bar + 5

Foo.bar is a property object which has a __get__ method. When you write something like

x = Foo()
print(x.bar)

the lookup for x.bar finds that type(x).bar has a __get__ method, and so the attribute lookup becomes equivalent to

type(x).bar.__get__(x, type(x))

which produces the value x._bar + 5.

The use of property as a decorator somewhat obscures the fact that bar is a property object. An equivalent defintion is

class Foo(object):
     def __init__(self):
         self._bar = 0

     bar = property(lambda self: self._bar + 5)

which shows more explicitly that you are creating a property object with the given lambda expression as the getter for that property, and binding the object to the class attribute bar.

The property class (along with instance methods, class methods, and static methods) is a specific application of Python's general descriptor protocol, which defines the behavior of class attributes with __get__, __set__, and/or __del__ methods.

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