Question on metaclasses

[Diez B. Roggisch]

Steffen Glückselig wrote:

> Hello,
> 
> I've been experimenting with metaclasses a bit (even though I am quite
> a newbie to python) and stumpled over the following problem in my code:
> 
> class Meta(type):
>   def __init__(cls, name, bases, dct):
>     for attr, value in dct.items():
>       if callable(value):
>         dct[attr] = wrapper(value)
> 
> wrapper adds debugging-information to methods of the class (at least
> that is my plan).
> 
> Using dct[attr] = wrapper(value) does not result in wrapped methods,
> though. Using setattr(cls, attr, wrapper(value)) creates the desired
> effect, though.
> 
> Why are the changes to dct not visible in the instantiated class? Is
> dct not the namespace of the class currently instantiated?

You don't use metaclasses correctly I believe. Usage should look like this:

class Foo(type):
    def __new__(cls, name, bases, dict):

        for k,v in [(k, v) for k,v in dict.items() if callable(v)]:
            cls.wrap(k,v,cls.get_directives(v), dict)

        return super(Foo, self).__new__(self, name, bases, dict)

Notice the __new__ instead of __init__, and the call to (actually, through
super) type.__new__

-- 
Regards,

Diez B. Roggisch