A newbie metaclass question

[Peter Otten]

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 2 line are not the same. Why? 

This particular difference has more to do with the descriptor protocol than
your custom metaclass. Instead of returning a class attribute unaltered, if
present, its __get__() method is called. E. g.:

>>> class A(object):
...     pass
...
>>> class Descr(object):
...     def __get__(self, *args):
...             print "args:", args
...             return 42
...
>>> A.attr = Descr()
>>> A.attr
args: (None, <class '__main__.A'>)
42
>>> A().attr
args: (<__main__.A object at 0x402ad62c>, <class '__main__.A'>)
42

And now let's make sure:

>>> hasattr(super(A), "__get__")
True

a 'super' object has a __get__() method, and is therefore free to give you
back whatever it chooses to. (It can detect whether it's called by an
instance or a class by checking its first argument).

You might have found some hints in the text you quote, but a detailed
explanation is here:

http://users.rcn.com/python/download/Descriptor.htm

> The metaclass of python is kinda difficult for me.

IIRC you were about to learn the OO basics just a few days ago. In that
situation I would put metaclasses _very_ low on my list of priorities. 

Peter