[Python-Dev] Class decorators

Phillip J. Eby pje at telecommunity.com
Tue Mar 28 06:41:50 CEST 2006

At 08:02 PM 3/27/2006 -0800, Guido van Rossum wrote:
>Just curious (and lazy): what magic is the implementation using that
>makes this work without a custom metaclass?

It registers a function as the __metaclass__ by poking it into the f_locals 
of the frame that's defining the class.  This function has two attributes 
that denote 1) the callback to be invoked, and 2) the previous metaclass, 
forming a linked list of callbacks.

When the class is created, this dummy metaclass function invokes the 
previous metaclass (or metaclass function) in the chain, then calls its 
callback on the result and returns that value, thus recursively unwinding 
the chain of advisors in the right order.  (i.e., the same order as the 
advisors were called in the class body).

The only part of the code that's even mildly complex is simulating Python's 
builtin rules for determining the default metaclass, so that when the 
recursion bottoms out you end up with an actual class with the right 
metaclass.  And that's not so much complex as tedious.

Anyway, the mechanism is orthogonal to the chosen metaclass, and thus can 
be used with classic classes or even ExtensionClasses if you're 
sufficiently masochistic.  :)  And AFAIK it's been in use by Zope 3 for 
some years now; in fact I think you were still working at Zope Corp. when 
Steve Alexander came up with the basic idea and I made the first practical 

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