[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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