[Python-Dev] Adding types.build_class for 3.3

[Mark Shannon]

Nick Coghlan wrote:
> On Thu, May 10, 2012 at 10:03 AM, Greg Ewing
> <greg.ewing at canterbury.ac.nz> wrote:
>> Python is not Java -- we have modules. Something should
>> only go in a class namespace if it somehow relates to
>> that particular class, and other classes could might
>> implement it differently. That's not the case with
>> build_class().

+1

> 
> Not true - you *will* get a type instance out of any sane call to
> type.define(). Technically, you could probably declare your metaclass
> such that you get a non-type object instead (just as you can with a
> class definition), but that means you're really just using an insanely
> convoluted way to make an ordinary function call. If you didn't want
> to invoke the full PEP 3115 find metaclass/prepare namespace/execute
> body/call metaclass dance, why would you be calling type.define
> instead of just calling the metaclass directly?

By attaching the 'define' object to type, then the descriptor protocol
causes problems if 'define' is a desriptor since type is its own 
metaclass. If it were a builtin-function, then there would be no problem.

A module-level builtin-function is more likely to be correct and seems
to me to be more Pythonic. Not that I'm a good judge of Pythonicness :)

Finally, could you remind me how the proposed type.define differs from 
builtins.__build_class__?
I can't see any difference (apart from parameter ordering and the extra 
name parameter in builtins.__build_class__).

Cheers,
Mark.