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

PJ Eby pje at telecommunity.com
Sat Apr 21 16:55:55 CEST 2012

(Sorry I'm so late to this discussion.)

I think that it's important to take into account the fact that PEP 3115
doesn't require namespaces to implement anything more than __setitem__ and
__getitem__ (with the latter not even needing to do anything but raise

Among other things, this means that .update() is right out as a
general-purpose solution to initializing a 3115-compatible class: you have
to loop and set items explicitly.  So, if we're providing helper functions,
there should be a helper that handles this common case by taking the
keywords (or perhaps an ordered sequence of pairs) and doing the looping
for you.

Of course, once you're doing that, you might as well implement it by
passing a closure into __build_class__...

More below:

On Sun, Apr 15, 2012 at 7:48 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:

> 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.

Thing is, though, if an implementation is dynamic enough to be capable of
supporting PEP 3115 *at all*  (not to mention standard exec/eval
semantics), it's going to have no problem mimicking __build_class__.

I mean, to implement PEP 3115 namespaces, you *have* to support exec/eval
with arbitrary namespaces.  From that, it's only the tiniest of steps to
wrapping that exec/eval in a function object to pass to __build_class__.

Really, making that function is probably the *least* of the troubles an
alternate implementation is going to have with supporting PEP 3115 (by
far).  Hell, supporting *metaclasses* is the first big hurdle an alternate
implementation has to get over, followed by the exec/eval with arbitrary

Personally, I think __build_class__ should be explicitly exposed and
supported, if for no other reason than that it allows one to re-implement
old-style __metaclass__ support in 2.x modules that rely on it...  and I
have a lot of those to port.  (Which is why I also think the convenience
API for PEP 3115-compatible class creation should actually call
__build_class__ itself.  That way, if it's been replaced, then the replaced
semantics would *also* apply to dynamically-created classes.)

IOW, there'd be two functions: one that's basically "call __build_class__",
and the other that's "call __build_class__ with a convenience function to
inject these values into the prepared dictionary".

Having other convenience functions that reimplement lower-level features
than __build_class__ (like the prepare thing) sounds like a good idea, but
I think we should encourage common cases to just call something that keeps
the __setitem__ issue out of the way.

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