@property 装饰器如何在 Python 中工作? [英] How does the @property decorator work in Python?

问题描述

我想了解内置函数 property 是如何工作的.令我困惑的是 property 也可以用作装饰器，但它仅在用作内置函数时才接受参数，而在用作装饰器时则不接受.

此示例来自文档:

C 类:def __init__(self):self._x = 无def getx(self):返回 self._xdef setx(self, value):self._x = 值def delx(自我):del self._xx = property(getx, setx, delx, 我是‘x’属性.")

property 的参数是 getx、setx、delx 和一个文档字符串.

在下面的代码中，property 用作装饰器.它的对象是 x 函数，但在上面的代码中，参数中没有对象函数的位置.

C 类:def __init__(self):self._x = 无@财产def x(self):"我是x"属性.""返回 [e-mail] x(self, value):self._x = 值@x.deleterdef x(self):del self._x

在这种情况下如何创建 x.setter 和 x.deleter 装饰器?

解决方案

property() 函数返回一个特殊的 描述符对象:

>>>财产()<0x10ff07940处的属性对象>

正是这个具有额外方法的对象:

<预><代码>>>>属性().getter<0x10ff07998处的属性对象的内置方法getter>>>>属性().setter<0x10ff07940处的属性对象的内置方法设置器>>>>属性().deleter<0x10ff07998处的属性对象的内置方法删除器>

这些也作为装饰器.他们返回一个新的属性对象:

<预><代码>>>>属性().getter(无)<0x10ff079f0处的属性对象>

这是旧对象的副本，但替换了其中一个功能.

记住，@decorator 语法只是语法糖；语法:

@propertydef foo(self): 返回 self._foo

其实和

意思一样

def foo(self): return self._foo富 = 财产(富)

so foo 函数被替换为 property(foo)，我们上面看到的是一个特殊的对象.然后，当您使用 @foo.setter() 时，您所做的是调用我上面向您展示的 property().setter 方法，它返回属性，但这次用 setter 函数替换为装饰方法.

以下序列还通过使用这些装饰器方法创建了一个完整的属性.

首先，我们创建一些函数和一个 property 对象，只有一个 getter:

<预><代码>>>>def getter(self): print('Get!')...>>>def setter(self, value): print('Set to {!r}!'.format(value))...>>>def deleter(self): 打印('删除！')...>>>道具 = 属性(吸气剂)>>>prop.fget 是吸气剂真的>>>prop.fset 是 None真的>>>prop.fdel 是 None真的

接下来我们使用.setter()方法来添加一个setter:

<预><代码>>>>prop = prop.setter(setter)>>>prop.fget 是吸气剂真的>>>prop.fset 是 setter真的>>>prop.fdel 是 None真的

最后我们使用 .deleter() 方法添加一个删除器:

<预><代码>>>>prop = prop.deleter(deleter)>>>prop.fget 是吸气剂真的>>>prop.fset 是 setter真的>>>prop.fdel 是删除器真的

最后但并非最不重要的是，property 对象充当 描述符对象，所以它有 .__get__(), .__set__() 和 .__delete__() 挂钩实例属性获取、设置和删除的方法:

<预><代码>>>>Foo类:通过...>>>prop.__get__(Foo(), Foo)得到！>>>prop.__set__(Foo(), 'bar')设置为酒吧"！>>>prop.__delete__(Foo())删除！

Descriptor Howto 包括 纯 Python 示例实现property() 类型:

<块引用>

class 属性:在 Objects/descrobject.c 中模拟 PyProperty_Type()"def __init__(self, fget=None, fset=None, fdel=None, doc=None):self.fget = fgetself.fset = fsetself.fdel = fdel如果 doc 是 None 并且 fget 不是 None:doc = fget.__doc__self.__doc__ = docdef __get__(self, obj, objtype=None):如果 obj 为 None:回归自我如果 self.fget 是 None:引发 AttributeError("不可读的属性")返回 self.fget(obj)def __set__(self, obj, value):如果 self.fset 是 None:引发 AttributeError("无法设置属性")self.fset(obj, value)def __delete__(self, obj):如果 self.fdel 是 None:raise AttributeError("无法删除属性")self.fdel(obj)def getter(self, fget):返回类型(self)(fget，self.fset，self.fdel，self.__doc__)def setter(self, fset):返回类型(self)(self.fget，fset，self.fdel，self.__doc__)def删除器(自我，fdel):返回类型(self)(self.fget，self.fset，fdel，self.__doc__)

I would like to understand how the built-in function property works. What confuses me is that property can also be used as a decorator, but it only takes arguments when used as a built-in function and not when used as a decorator.

This example is from the documentation:

class C:
    def __init__(self):
        self._x = None

    def getx(self):
        return self._x
    def setx(self, value):
        self._x = value
    def delx(self):
        del self._x
    x = property(getx, setx, delx, "I'm the 'x' property.")

property's arguments are getx, setx, delx and a doc string.

In the code below property is used as a decorator. The object of it is the x function, but in the code above there is no place for an object function in the arguments.

class C:
    def __init__(self):
        self._x = None

    @property
    def x(self):
        """I'm the 'x' property."""
        return self._x

    @x.setter
    def x(self, value):
        self._x = value

    @x.deleter
    def x(self):
        del self._x

How are the x.setter and x.deleter decorators created in this case?

解决方案

The property() function returns a special descriptor object:

>>> property()
<property object at 0x10ff07940>

It is this object that has extra methods:

>>> property().getter
<built-in method getter of property object at 0x10ff07998>
>>> property().setter
<built-in method setter of property object at 0x10ff07940>
>>> property().deleter
<built-in method deleter of property object at 0x10ff07998>

These act as decorators too. They return a new property object:

>>> property().getter(None)
<property object at 0x10ff079f0>

that is a copy of the old object, but with one of the functions replaced.

Remember, that the @decorator syntax is just syntactic sugar; the syntax:

@property
def foo(self): return self._foo

really means the same thing as

def foo(self): return self._foo
foo = property(foo)

so foo the function is replaced by property(foo), which we saw above is a special object. Then when you use @foo.setter(), what you are doing is call that property().setter method I showed you above, which returns a new copy of the property, but this time with the setter function replaced with the decorated method.

The following sequence also creates a full-on property, by using those decorator methods.

First we create some functions and a property object with just a getter:

>>> def getter(self): print('Get!')
... 
>>> def setter(self, value): print('Set to {!r}!'.format(value))
... 
>>> def deleter(self): print('Delete!')
... 
>>> prop = property(getter)
>>> prop.fget is getter
True
>>> prop.fset is None
True
>>> prop.fdel is None
True

Next we use the .setter() method to add a setter:

>>> prop = prop.setter(setter)
>>> prop.fget is getter
True
>>> prop.fset is setter
True
>>> prop.fdel is None
True

Last we add a deleter with the .deleter() method:

>>> prop = prop.deleter(deleter)
>>> prop.fget is getter
True
>>> prop.fset is setter
True
>>> prop.fdel is deleter
True

Last but not least, the property object acts as a descriptor object, so it has .__get__(), .__set__() and .__delete__() methods to hook into instance attribute getting, setting and deleting:

>>> class Foo: pass
... 
>>> prop.__get__(Foo(), Foo)
Get!
>>> prop.__set__(Foo(), 'bar')
Set to 'bar'!
>>> prop.__delete__(Foo())
Delete!

The Descriptor Howto includes a pure Python sample implementation of the property() type:

class Property:
    "Emulate PyProperty_Type() in Objects/descrobject.c"

    def __init__(self, fget=None, fset=None, fdel=None, doc=None):
        self.fget = fget
        self.fset = fset
        self.fdel = fdel
        if doc is None and fget is not None:
            doc = fget.__doc__
        self.__doc__ = doc

    def __get__(self, obj, objtype=None):
        if obj is None:
            return self
        if self.fget is None:
            raise AttributeError("unreadable attribute")
        return self.fget(obj)

    def __set__(self, obj, value):
        if self.fset is None:
            raise AttributeError("can't set attribute")
        self.fset(obj, value)

    def __delete__(self, obj):
        if self.fdel is None:
            raise AttributeError("can't delete attribute")
        self.fdel(obj)

    def getter(self, fget):
        return type(self)(fget, self.fset, self.fdel, self.__doc__)

    def setter(self, fset):
        return type(self)(self.fget, fset, self.fdel, self.__doc__)

    def deleter(self, fdel):
        return type(self)(self.fget, self.fset, fdel, self.__doc__)

这篇关于@property 装饰器如何在 Python 中工作?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！