could ildg wrote:
> When I try to learn metaclass of python by article at this place:
> http://www.python.org/2.2/descrintro.html#metaclasses,
> I changed the autosuper example a little as below:
> <code>
> class autosuper(type):
> def __init__(cls,name,bases,dict):
> super(autosuper,cls).__init__(name,bases,dict)
> setattr(cls,"_%s__super" % name, super(cls))
> print "in metaclass: ", super(cls)
>
> class A:
> __metaclass__ = autosuper
> def meth(self):
> print "in class A: ", self.__super
>
> a=A()
> a.meth()
> </code>
> The result is as below:
> in metaclass: <super: <class 'A'>, NULL>
> in class A: <super: <class 'A'>, <A object>>
The reason for this is that the super object is a descriptor. This
means that
self.__super
actually calls something like:
type(self).__super.__get__(self)
So the difference in the two is that the "in metaclass" print references
the unbound super object, and the "in class A" print references the
bound super object.
A very simple demonstration of this:
py> class C(object):
... pass
...
py> C.sup = super(C)
py> C.sup
<super: <class 'C'>, NULL>
py> C().sup
<super: <class 'C'>, <C object>>
py> C.sup.__get__(C())
<super: <class 'C'>, <C object>>
Note that we get the "NULL" output when we ask the *class* for the "sup"
object. We get the "C object" output when we ask an *instance* for the
"sup" object. The example using __get__ is basically just the explicit
version of the one that doesn't use __get__.
HTH,
STeVe
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