[Python-3000] Metaclasses in Py3K (original) (raw)

Steven Bethard steven.bethard at gmail.com
Mon Dec 18 07:52:35 CET 2006

On 12/17/06, Josiah Carlson <jcarlson at uci.edu> wrote:

Say I have a metaclass X. Say that I've been using metaclass X since Python 2.2 days. One day I hear that shortly after Py3k, metaclass is going away and I need to start using some other syntax (say the metaclass= syntax for arguments sake). So what do I do?

class Foo(A, B, metaclass=X): ... And I think that I have fixed my implementation for Py3k+, nevermind that it doesn't work in Python 2.x anymore, I'm a forward thinker and I like the new syntax. X implemented a class registry, so when X is called twice (once for setup, and once for finalization), now the class gets registered twice, or perhaps I get a crash because my metaclass doesn't have a default dict argument that discriminates between the two cases (using Steven's semantics). Ok, so now I need to adjust my metaclass to check to see whether it is doing the setup or finalization step. That's fine, but now I've had to modify my metaclass, something that I was told I wouldn't need to do, and with metaclass going away (presumably, otherwise would we create a new syntax?), I'm going to need to rewrite all of my metaclasses when I switch syntaxes. But I thought you said I wouldn't need to rewrite all of my metaclasses?

FWLIW, avoiding having to rewrite code was not a goal I was striving for. You're already going to have to rewrite for 3.0 any of your code that uses, for example, dict.itervalues(), dict.iteritems(), etc., and I thought people had accepted that because there is a way to write that code that should work in both 2.X and 3.0 (e.g. using only iter(dict) or dict.values(), etc.)

So, for me, the goal was to make sure that there was a way to write code that worked in both 2.X and 3.0. That is, I didn't mind if you had to rewrite some 2.X code to make it future compatible, as long as it was possible to write such code.

STeVe

I'm not in-sane. Indeed, I am so far out of sane that you appear a tiny blip on the distant coast of sanity. --- Bucky Katt, Get Fuzzy

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