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

Jim Jewett jimjjewett at gmail.com
Tue Dec 12 18:09:11 CET 2006

On 12/8/06, Guido van Rossum <guido at python.org> wrote:

I've been thinking about this too, and I think it's reasonable to let the metaclass provide the dict to be used as locals.

Is this something that could be done in 2.6, so long as the metaclass does happen to be set before the class statement?

This is easier when the metaclass is given in the class header, ... I'd rather innovate a bit more and add keyword arguments to the class header,

class C(B1, B2, metaclass=Foo): ...

Something about that seems to be begging for either confusion (when can/must I use a keyword?) or a mess (which keywords can I use? What code handles them?)

Would this be a reasonable use of a with- block?

Better yet, could it be done with a class decorator?

@meta(Foo)
class C(B1, B2): ...

Typically, the decorator wouldn't be called until too late, but the decorator would be created (specialized to Foo) in time. Could it open a with-context that resets the default metaclass for the duration of the class statement?

(Strangely enough, when I tried this out, a with- block that set the module-global metaclass seemed to work for classic classes but not for new-style classes. I didn't investigate beyond that. Code below the .sig for those who care.)

-jJ

"""with-statement for metaclass"""

from future import with_statement

from contextlib import contextmanager

@contextmanager def meta(mcls): """Sets mcls as the default metaclass for the duration of the with-block""" global metaclass try: oldmeta = metaclass except NameError: oldmeta = None metaclass = mcls yield if oldmeta is None: del metaclass else: metaclass = oldmeta

########## testing ############

class ChattyType(type): def new(cls, name, bases, dct): print "Allocating memory for class", name return type.new(cls, name, bases, dct) def init(cls, name, bases, dct): print "Init'ing (configuring) class", name super(ChattyType, cls).init(name, bases, dct)

class N1(object): pass class O1: pass

with meta(ChattyType): class N2(object): pass class O2: pass class Nesting1(object): class Nesting2(N2): class NestingO(O2): class NestingOb: class NestingNdeep(object): pass

class N3(object): pass class O3: pass

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