The override applies to descriptors that are part of the class __dict__
.
Python will always look up type(instance).__dict__[attributename].__get__(instance, type(instance))
, and will not use instance.__dict__
to search for a instance-override.
Here is an example using a contrived Descriptor
class and a property (which is a descriptor with a __get__
and a __set__
:
>>> 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'
As you can see, even though I add a spam
entry in the instance __dict__
, it is completely ignored and the Foo.spam
property is used still. Python is ignoring the instance __dict__
because the spam
property defines both __get__
and a __set__
.
If you use a descriptor that doesn't define a __set__
the override works (but it's __get__
is not called:
>>> class Foo(object):
... desc = Descriptor('Class-stored descriptor')
...
>>> Foo.desc
Getting Class-stored descriptor, with instance None, class <class '__main__.Foo'>
>>> Foo().desc
Getting Class-stored descriptor, with instance <__main__.Foo object at 0x1018df510>, class <class '__main__.Foo'>
>>> foo = Foo()
>>> foo.__dict__['desc'] = Descriptor('Instance-stored descriptor')
>>> foo.desc
<__main__.Descriptor object at 0x1018df1d0>
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…