@property装饰器在Python中如何工作？

Question

我想了解内置函数的property工作原理。令我感到困惑的是，property它还可以用作装饰器，但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参数。

这个例子来自文档：

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的论点是getx，setx，delx和文档字符串。

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

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

在这种情况下，x.setter和x.deleter装饰器是如何创建的？

Answer 1

该property()函数返回一个特殊的描述符对象：>>> property()<property object at 0x10ff07940>正是这种对象有额外的方法：>>> 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>这些充当装饰过。他们返回一个新的属性对象：>>> property().getter(None)<property object at 0x10ff079f0>那是旧对象的副本，但是替换了其中一个功能。记住，@decorator语法只是语法糖。语法：@propertydef foo(self): return self._foo确实与def foo(self): return self._foofoo = property(foo)因此foo该函数被替换了property(foo)，我们在上面看到的是一个特殊的对象。然后，当您使用时@foo.setter()，您正在做的事情就是调用property().setter我上面显示的方法，该方法将返回该属性的新副本，但这一次是将setter函数替换为装饰方法。下面的序列还通过使用那些装饰器方法创建了一个全开属性。首先，我们创建一些函数和一个property仅带有吸气剂的对象：>>> def getter(self): print('Get!')...&nbsp;>>> def setter(self, value): print('Set to {!r}!'.format(value))...&nbsp;>>> def deleter(self): print('Delete!')...&nbsp;>>> prop = property(getter)>>> prop.fget is getterTrue>>> prop.fset is NoneTrue>>> prop.fdel is NoneTrue接下来，我们使用该.setter()方法添加一个setter：>>> prop = prop.setter(setter)>>> prop.fget is getterTrue>>> prop.fset is setterTrue>>> prop.fdel is NoneTrue最后，我们使用以下.deleter()方法添加删除器：>>> prop = prop.deleter(deleter)>>> prop.fget is getterTrue>>> prop.fset is setterTrue>>> prop.fdel is deleterTrue最后但并非最不重要的，property对象作为一个描述符对象，所以它有.__get__()，.__set__()和.__delete__()方法挂接到实例属性获取，设置和删除：>>> class Foo: pass...&nbsp;>>> prop.__get__(Foo(), Foo)Get!>>> prop.__set__(Foo(), 'bar')Set to 'bar'!>>> prop.__delete__(Foo())Delete!Descriptor Howto包括以下类型的纯Python示例实现property()：class Property:&nbsp; &nbsp; "Emulate PyProperty_Type() in Objects/descrobject.c"&nbsp; &nbsp; def __init__(self, fget=None, fset=None, fdel=None, doc=None):&nbsp; &nbsp; &nbsp; &nbsp; self.fget = fget&nbsp; &nbsp; &nbsp; &nbsp; self.fset = fset&nbsp; &nbsp; &nbsp; &nbsp; self.fdel = fdel&nbsp; &nbsp; &nbsp; &nbsp; if doc is None and fget is not None:&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; doc = fget.__doc__&nbsp; &nbsp; &nbsp; &nbsp; self.__doc__ = doc&nbsp; &nbsp; def __get__(self, obj, objtype=None):&nbsp; &nbsp; &nbsp; &nbsp; if obj is None:&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; return self&nbsp; &nbsp; &nbsp; &nbsp; if self.fget is None:&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; raise AttributeError("unreadable attribute")&nbsp; &nbsp; &nbsp; &nbsp; return self.fget(obj)&nbsp; &nbsp; def __set__(self, obj, value):&nbsp; &nbsp; &nbsp; &nbsp; if self.fset is None:&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; raise AttributeError("can't set attribute")&nbsp; &nbsp; &nbsp; &nbsp; self.fset(obj, value)&nbsp; &nbsp; def __delete__(self, obj):&nbsp; &nbsp; &nbsp; &nbsp; if self.fdel is None:&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; raise AttributeError("can't delete attribute")&nbsp; &nbsp; &nbsp; &nbsp; self.fdel(obj)&nbsp; &nbsp; def getter(self, fget):&nbsp; &nbsp; &nbsp; &nbsp; return type(self)(fget, self.fset, self.fdel, self.__doc__)&nbsp; &nbsp; def setter(self, fset):&nbsp; &nbsp; &nbsp; &nbsp; return type(self)(self.fget, fset, self.fdel, self.__doc__)&nbsp; &nbsp; def deleter(self, fdel):&nbsp; &nbsp; &nbsp; &nbsp; return type(self)(self.fget, self.fset, fdel, self.__doc__)

Answer 2

文档说，这只是创建只读属性的捷径。所以@propertydef x(self):&nbsp; &nbsp; return self._x相当于def getx(self):&nbsp; &nbsp; return self._xx = property(getx)