python数据描述符_python数据和非数据描述符

覆盖适用于属于类__dict__的描述符.

Python将始终查找类型(实例).__ dict __ [attributename] .__ get __(实例,类型(实例)),并且不会使用实例.__ dict__来搜索实例覆盖.

下面是一个使用设计的Descriptor类和属性的示例(它是一个带有__get__和__set__的描述符：

>>> class Descriptor(object):

... def __init__(self, name):

... self.name = name

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

... print 'Getting %s, with instance %r, class %r' % (self.name, instance, cls)

...

>>> class Foo(object):

... _spam = 'eggs'

... @property

... def spam(self):

... return self._spam

... @spam.setter

... def spam(self, val):

... self._spam = val

...

>>> Foo().spam

'eggs'

>>> foo = Foo()

>>> foo.__dict__['spam'] = Descriptor('Override')

>>> foo.spam

'eggs'

正如您所看到的,即使我在实例__dict__中添加了垃圾邮件条目,它也会被完全忽略,并且仍然使用Foo.spam属性. Python忽略了实例__dict__,因为spam属性定义了__get__和__set__.

如果您使用未定义__set__的描述符,则覆盖可以工作(但不会调用__get__：

>>> class Foo(object):

... desc = Descriptor('Class-stored descriptor')

...

>>> Foo.desc

Getting Class-stored descriptor, with instance None, class

>>> Foo().desc

Getting Class-stored descriptor, with instance <__main__.foo object at>, class

>>> foo = Foo()

>>> foo.__dict__['desc'] = Descriptor('Instance-stored descriptor')

>>> foo.desc