[Python-Dev] Providing a mechanism for PEP 3115 compliant dynamic class creation (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Sun Apr 15 13:48:06 CEST 2012

/me pages thoughts from 12 months ago back into brain...

On Sun, Apr 15, 2012 at 7:36 PM, Daniel Urban <urban.dani+py at gmail.com> wrote:

On Tue, Apr 19, 2011 at 16:10, Nick Coghlan <ncoghlan at gmail.com> wrote:

Initially I was going to suggest making buildclass part of the language definition rather than a CPython implementation detail, but then I realised that various CPython specific elements in its signature made that a bad idea. Are you referring to the first 'func' argument? (Which is basically the body of the "class" statement, if I'm not mistaken).

Yup, I believe that was my main objection to exposing build_class directly. There's no obligation for implementations to build a throwaway function to evaluate a class body.

prepare also needs the name and optional keyword arguments.  So it probably should be something like "operator.prepare(name, bases, metaclass, **kw)". But this way it would need almost the same arguments as buildclass(func, name, *bases, metaclass=None, **kwds).

True.

The correct idiom for dynamic type creation in a PEP 3115 world would then be:

 from operator import prepare  cls = type(name, bases, prepare(type, bases)) Thoughts? When creating a dynamic type, we may want to do it with a non-empty namespace. Maybe like this (with the extra arguments mentioned above): from operator import prepare ns = prepare(name, bases, type, **kwargs) ns.update(myns)  # add the attributes we want cls = type(name, bases, ns) What about an "operator.buildclass(name, bases, ns, **kw)" function? It would work like this: def buildclass(name, bases, ns, **kw): metaclass = kw.pop('metaclass', type) pns = prepare(name, bases, metaclass, **kw) pns.update(ns) return metaclass(name, bases, pns) (Where 'prepare' is the same as above). This way we wouldn't even need to make 'prepare' public, and the new way to create a dynamic type would be: from operator import buildclass cls = buildclass(name, bases, ns, **mykwargs)

No, I think we would want to expose the created namespace directly - that way people can use update(), direct assigment, exec(), eval(), or whatever other mechanism they choose to handle the task of populating the namespace. However, a potentially cleaner way to do that might be offer use an optional callback API rather than exposing a separate public prepare() function. Something like:

def build_class(name, bases=(), kwds=None, eval_body=None):
    metaclass, ns = _prepare(name, bases, kwds)
    if eval_body is not None:
        eval_body(ns)
    return metaclass(name, bases, ns)

Cheers, Nick.

-- Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia

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