[Python-Dev] Class decorators vs metaclasses

Eyal Lotem eyal.lotem at gmail.com
Fri Nov 4 23:33:29 CET 2005

I have a few claims, some unrelated, and some built on top of each
other.  I would like to hear your responses as to which are
convincing, which arne't, and why. I think that if these claims are
true, Python 3000 should change quite a bit.

A. Metaclass code is black magic and few understand how it works,
while decorator code is mostly understandable, even by non-gurus.

B. One of Decorators' most powerful features is that they can
mixed-and-matched, which makes them very powerful for many purposes,
while metaclasses are exclusive, and only one can be used.  This is
especially problematic as some classes may assume their subclasses
must use their respective metaclasses.  This means classdecorators are
strictly more powerful than metaclasses, without cumbersome
convertions between metaclass mechanisms and decorator mechanisms.

C. Interesting uses of classdecorators are allowing super-calling
without redundantly specifying the name of your class, or your

D. Python seems to be incrementally adding power to the core language
with these features, which is great, but it also causes significant
overlapping of language features, which I believe is something to
avoid when possible.  If metaclasses are replaced with class
decorators, then suddenly inheritence becomes a redundant feature.

E. If inheritence is a redundant feature, it can be removed and an
"inherit" class decorator can be used.  This could also reduce all the
__mro__ clutter from the language along with other complexities, into
alternate implementations of the inherit classdecorator.

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