6. Python描述器¶
tags： Python
[TOC]
6.1. 要点¶
- 描述器只对新式类起作用；
- 如果访问ojb.attr，其中attr既是obj的实例属性，也是obj的一个描述器时，到底访问的哪个attr取决于attr描述器的类型。优先级是：资源描述器 -> 实例字典 -> 非资源描述器。请看示例一：
- 把描述符放在类的层次上：为了让描述符能够正常工作，它们必须定义在类的层次上。如果你不这么做，那么Python无法自动为你调用__get和set__方法。
- 确保实例的数据只属于实例本身。
# 示例一
# 非资源描述器
class cached_property(object):
def __init__(self, func, name=None, doc=None):
self.__name__ = name or func.__name__
self.__module__ = func.__module__
self.__doc__ = doc or func.__doc__
self.func = func
def __get__(self, obj, type=None):
if obj is None:
return self
value = obj.__dict__.get(self.__name__, _missing)
if value is _missing:
value = self.func(obj)
obj.__dict__[self.__name__] = value
return value
# 资源描述器
class cached_property_2(object):
def __init__(self, func, name=None, doc=None):
self.__name__ = name or func.__name__
self.__module__ = func.__module__
self.__doc__ = doc or func.__doc__
self.func = func
def __get__(self, obj, type=None):
if obj is None:
return self
value = obj.__dict__.get(self.__name__, _missing)
if value is _missing:
value = self.func(obj)
obj.__dict__[self.__name__] = value
return value
def __set__(self, obj, value):
pass
class Foo(object):
def __init__(self):
self.f1 = "name f1"
self.f2 = "instance f2"
self.tt = cached_property(test1)
pass
@cached_property
def f1(self):
print 'first f1,,',
result = 'this is result'
return result
# f1 = cached_property(f1)
@cached_property_2
def f2(self):
print 'first f2,,',
return "description f2"
def test1():
f = Foo()
print f.f1 # first calculate this is result
print f.f1 # this is result
print f.f2
print f.f2
请参考：
http://pyzh.readthedocs.io/en/latest/Descriptor-HOW-TO-Guide.html
http://blog.csdn.net/huithe/article/details/7484606
http://hbprotoss.github.io/posts/python-descriptor.html
http://www.jianshu.com/p/250f0d305c35
https://segmentfault.com/a/1190000004238416
http://strawhatfy.github.io/2015/04/21/python-attribute-lookup/
http://www.jianshu.com/p/250f0d305c35
http://pyzh.readthedocs.io/en/latest/Descriptor-HOW-TO-Guide.html