how can i use '__set__'
<pre><code>class A(object):
    def __get__(self, instance, owner):#why i can't find argument 'key',where is 'key'
        #print ower.instance
        print instance,owner
    def __set__(self,instance,value):
        instance=value

class X(object):
    a = A()

xx=X()
xx.a='aaa'
print xx.a#None
</code></pre>

<p>thanks</p>
