[Python-Dev] Providing a mechanism for PEP 3115 compliant dynamic class creation

[Nick Coghlan]

/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 __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).

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(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)

Cheers,
Nick.

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