Python property，属性

參考资料

http://www.ibm.com/developerworks/library/os-pythondescriptors/

顾名思义，property用于生成一个属性。通过操作这个属性。能够映射为对某些函数的操作，类似于C#。

形式为

pvar = propery(get_func, set_func, del_fun, doc_func);

在get。set，del，doc的时候分别调用对应的函数；

尝试这样定义

>>> aa = property(lambda: "hello", lambda x: print(x))

>>> aa

<property object at 0x7fdcd9ecf9a8>

>>> aa = 1

看来property仅仅有在class中才有意义（看后文）

>>> class Test:

... aa = property(lambda self: "hello", lambda
self,x: print(x))

...

>>> t = Test()

>>> t.aa

'hello'

>>> t.aa = (1,2,2)

(1, 2, 2)

property的神奇面纱

事实上propery并非一个真正的函数，而是一个类

>>> type(property)

<class 'type'>

而普通的函数为

>>> def f():

... pass

...

>>> type(f)

<class 'function'>

property类实现了__get__, __set__, __delete__方法，这3个方法合在一起，就定义了描写叙述符规则，实现了当中不论什么一个方法的对象就叫描写叙述符（descriptor）。描写叙述符的特殊之处在于它使怎样被訪问的。比方，程序读取一个属性时。假设该属性被绑定到实现了__get__方法的对象上。那么就会调用__get__方法（返回其结果），而不是简单的返回该对象。

class Something:

... def __get__(self, instance, owner):

... print(instance)

... print(owner)

... return "__get__"

... def __set__(self, instance, name):

... print(instance)

... print(name)

... print("__set__")

>>> class Holder:

... s = Something()

...

>>> h = Holder()

>>> h.s

<__main__.Holder object at 0x7fdcd9ed8518>

<class '__main__.Holder'>

'__get__'

>>> h.s = 3

<__main__.Holder object at 0x7fdcd9ed8518>

__set__

Something的__get__ 和 __set__方法的參数instance，相应了Something对象所所绑定的对象；能够想象。Property是通过instance调用传入当中的get, set, del, doc。

须要注意的是：描写叙述符必须被赋值给类属性，而不是对象实例属性，才有效；

>>> class Something:

... pass

>>> setattr(Something, 'aa', property(lambda self: "hello", lambda self, x: print(x)))

>>> t.aa

'hello'

>>> t.aa = 3

而设置在对象实例上，则无效果：

>>> setattr(t, 'aa', property(lambda self: "hello", lambda self, x: print(x)))

>>> setattr(Something, 'aa', None)

>>> t.aa

<property object at 0x7fdcd9edf4f8>

>>> t.aa = 3

另外，还能够通过注解（Annotation）的方式定义property

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

http://www.ibm.com/developerworks/library/os-pythondescriptors/给了一种动态创建Property的方法

class Person(object):

def addProperty(self, attribute):

# create local setter and getter with a particular attribute name

getter = lambda self: self._getProperty(attribute)

setter = lambda self, value: self._setProperty(attribute, value)

# construct property attribute and add it to the class

setattr(self.__class__, attribute, property(fget=getter, \

fset=setter, \

doc="Auto-generated method"))

def _setProperty(self, attribute, value):

print "Setting: %s = %s" %(attribute, value)

setattr(self, '_' + attribute, value.title())

def _getProperty(self, attribute):

print "Getting: %s" %attribute

return getattr(self, '_' + attribute)