On Sun, Apr 15, 2012 at 13:48, Nick Coghlan firstname.lastname@example.org wrote:
/me pages thoughts from 12 months ago back into brain...
Sorry about that, I planned to do this earlier...
On Sun, Apr 15, 2012 at 7:36 PM, Daniel Urban email@example.com wrote:
On Tue, Apr 19, 2011 at 16:10, Nick Coghlan firstname.lastname@example.org wrote:
Initially I was going to suggest making __build_class__ 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 __build_class__(func, name, *bases, metaclass=None, **kwds).
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))
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(my_ns) # add the attributes we want cls = type(name, bases, ns)
What about an "operator.build_class(name, bases, ns, **kw)" function? It would work like this:
def build_class(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 build_class cls = build_class(name, bases, ns, **my_kwargs)
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)
That seems more flexible indeed. I will try to make a patch next week, if that's OK.