[Python-3000] Metaclasses in Python 3000: Draft 2 (original) (raw)

Talin talin at acm.org
Wed Mar 14 21:07:58 CET 2007

Guido van Rossum wrote:

On 3/14/07, Talin <talin at acm.org> wrote:

PEP: xxx Title: Metaclasses in Python 3000 Checked in as PEP 3115. I fixed the two typos that were noted so far (missing comma and inconsistency in name of first arg to prepare; I renamed the latter metacls) and cleaned up one case of extra whitespace (.append( key ) -> .append(key)). Now on to the content:

def prepareclass(name, *bases, metaclass=type, **kwargs): prepare = getattr(metaclass, 'prepare', None) if prepare is not None: return prepare(name, bases, **kwargs) else: return dict() This is missing the extraction of the metaclass default from the bases. It's probably okay to default metaclass to None and add this code to the body: if metaclass is None: metaclass = computedefaultmetaclass(bases) type. It does not need to implement the full dictionary interface; only the ability to insert items and retrieve them are required. (Note: double check that this is true). As was mentioned, getitem, setitem and delitem are used. No other APIs are, unless the dict is accessed via locals(), or unless dir() is used. IMO it's up to the prepare function to provide the features that it wants to support; if it doesn't want to support deletions, that is between the metaclass and the users; the language spec doesn't have to say anything about this. # The custom dictionary class membertable(dict): def init(self): Despite this just being an example, I'd like for the membertable class to be defined outside the OrderedClass class. it also probably ought to have a conforming name, i.e. MemberTable. Another good suggestion was to simply use an ordered dict for all classes, and skip the whole 'custom dict' mechanism. This was based on the observation that most use cases for a custom dict were for the purposes of preserving order information. However, this idea has two drawbacks, first because it means that an ordered dict implementation would have to be added to the set of built-in types in Python, and second because it would impose a slight speed (and complexity) penalty on all class declarations. I think this rebuttal isn't strong enough; I'm sure there will be use cases where a custom prepare method can solve a problem that a standard ordered dict couldn't.

This all looks good to me - I think I still have permission to check in changes to the PEPs dir, want me to go ahead and make the changes directly?

-- Talin

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