[Python-3000] super(), class decorators, and PEP 3115

Phillip J. Eby pje at telecommunity.com
Tue May 1 01:50:09 CEST 2007

At 03:54 PM 4/30/2007 -0700, Guido van Rossum wrote:
>On 4/30/07, Phillip J. Eby <pje at telecommunity.com> wrote:
>>At 12:17 PM 4/30/2007 -0700, Guido van Rossum wrote:
>> >Assuming class decorators are added, can't you do all of this using a
>> >custom metaclass?
>>The only thing I need for the GF PEP is a way for a method decorator to get
>>a callback after the class is created, so that overloading will work
>>correctly in cases where overloaded methods are defined in a subclass.
>I still don't understand why you can't tell the users "for this to
>work, you must use my special magic super-duper metaclass defined
>*here*". Surely a sufficiently advanced metaclass can pull of this
>kind of magic in its __init__ method? If not a metaclass, then a
>super-duper decorator. Or what am I missing?

Metaclasses don't mix well.  If the user already has a metaclass, they'll 
have to create a custom subclass, since Python doesn't do auto-combination 
of metaclasses (per the "Putting Metaclasses to Work" book).  This makes 
things messy, especially if the user doesn't *know* they're using a 
metaclass already (e.g., they got one by inheritance).

For the specific use case I'm concerned about, it's like "super()" in that 
a function defined inside a class body needs to know what class it's 
in.  (Actually, it's the decorator that needs to know, and it ideally needs 
to know as soon as the class is defined, rather than waiting until a call 
occurs later.)

As with "super()", this really has nothing to do with the class.  It would 
make about as much sense as having a metaclass or class decorator called 
``SuperUser``; i.e., it would work, but it's just overhead for the user.

So, if there ends up being a general way to access that "containing class" 
from a function decorator, or at least to get a callback once the class is 
defined, that's all I need for this use case that can't reasonably be 
handled by a normal metaclass.

Note, too, that the such a hook would also allow you to make classes into 
ABCs through the presence of an @abstractmethod, without also having to 
inherit from Abstract or set an explicit metaclass.  (Unless of course you 
prefer to have the abstractness called out up-front...  but then that 
explicitness goes out the window as soon as you e.g. sublcass Sequence from 

>But I don't understand how a __metaclass__ hack can use a class decorator.

The __metaclass__ hack is used in Python 2.x to dynamically *add* class 
decorators while the class suite is being executed, that will be called 
*after* the class is created.  A function decorator (think of your 
@abstractmethod, for example) would monkeypatch the metaclass so it gets a 
crack at class after it's created, without the user having to explicitly 
set up the metaclass (or merge any inherited metaclasses).

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