[Python-3000] PEP for Metaclasses in Python 3000 (original) (raw)

Steven Bethard steven.bethard at gmail.com
Fri Mar 9 22:23:39 CET 2007

On 3/9/07, Talin <talin at acm.org> wrote:

PEP: xxx Title: Metaclasses in Python 3000

Thanks for writing this.

This attribute is a method named metacreate, which is invoked before the evaluation of the class body, and which has the following form:

classdict = metaclass.metacreate(name, bases, keywords) [snip] There were some objections in the discussion to the 'two-phase' creation process, where the metaclass is invoked twice, once to create the class dictionary and once to 'finish' the class. Some people felt that these two phases should be completely separate, in that there ought to be separate syntax for specifying the custom dict as for specifying the metaclass. However, in most cases, the two will be intimately tied together, and the metaclass will most likely have an intimate knowledge of the internal details of the class dict. Requiring the programmer to insure that the correct dict type and the correct metaclass type are used together creates an additional and unneeded burden on the programmer.

I think it would really help to give some example code so that people can see why one way is easier than the other. I gather for using an ordered dict, the two alternatives would look something like::

# using __metacreate__ class/static method
class meta(type):
    @staticmethod
    def __metacreate__(name, bases, keywords):
        return ordereddict()
    def __init__(cls, name, bases, bodydict):
        for key, value in bodydict.items():     # I know this is in order
            ...
class C(metaclass=meta):
    ...

# using separate callables
def meta(name, bases, bodydict):
    for key, value in bodydict.items():     # I know this is in order
        ...
class C(metaclass=meta, bodydict=ordereddict):
    ...

But it would be nice to fill in those ellipses with code that does something useful.

I'd rather __metacreate__ be called something like __createdict__, __creationdict__, __metadict__, __getmetadict__, etc. where it's clearer that the purpose is to create a dict object.

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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