Python饰器@property

时间:2022-05-23 小朋友2D 人气:0

最初的声明方式

在没有@property修饰的情况下，需要分别声明get、set、delete函数，然后初始化property类，将这些方法加载进property中

class C持有property的实例化对象x

对外表现出来C().x时，实际上是调用C()中的x（property类）中设置的fset，fget，fdel，分别对应getx，setx，delx

C真正持有的x，是self._x被隐藏起来了

class C(object):
    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类结合x = property(getx, setx, delx, "I'm the 'x' property.")与property的__init__()可以发现property接受四个参数

fget，用于获取属性值，

fset，用于设置属性值

fdel，用于删除属性

doc，属性的介绍

可以单独设置fget、fset、fdel…

x = property，x.getter(getx)，x.setter(setx)，x.deleter(delx)

class property(object):
    
    def deleter(self, *args, **kwargs): # real signature unknown
        """ Descriptor to change the deleter on a property. """
        pass

    def getter(self, *args, **kwargs): # real signature unknown
        """ Descriptor to change the getter on a property. """
        pass

    def setter(self, *args, **kwargs): # real signature unknown
        """ Descriptor to change the setter on a property. """
        pass

    def __delete__(self, *args, **kwargs): # real signature unknown
        """ Delete an attribute of instance. """
        pass

    def __getattribute__(self, *args, **kwargs): # real signature unknown
        """ Return getattr(self, name). """
        pass

    def __get__(self, *args, **kwargs): # real signature unknown
        """ Return an attribute of instance, which is of type owner. """
        pass

    def __init__(self, fget=None, fset=None, fdel=None, doc=None): # known special case of 
        pass

使用装饰器的声明方式

需要注意，装饰器只是一个python的语法糖，可以拆解成普通使用方法，如property(getx)

@property创建了一个实例x，对于def x(self)实际上是C类持有x = property(fget=x)

因此，x.setter方法指向的是property.setter，也是起到装饰器效果x.setter(x)（注意，前者x是property实例x，后者x是def x(self, value)函数），x.deleter同理

class C(object):
    @property
    def x(self):
        "I am the 'x' property."
        return self._x
    
    @x.setter
    def x(self, value):
        self._x = value
        
    @x.deleter
    def x(self):
        del self._x

为什么property实例化后的名字与属性名一致？

换种问法就是为什么x = property(...)

可以认为是

attributes_and_methods = {
    x.__name__: property(x), //声明C类持有property实例
    #...
}
C = type('C', (object,), attributes_and_methods)

使用装饰器的调用过程

执行C().x时，调用的是C().x（property）绑定的fget方法，用过__get__唤醒，setter、deleter同理

class property(object):
    
    #...
    def __init__(self, fget=None, fset=None, fdel=None, doc=None):
        self.fget = fget
        self.fset = fset
        self.fdel = fdel
        ...

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

总结

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