[Python-Dev] Adding types.build_class for 3.3

[Mark Shannon]

Nick Coghlan wrote:
> On Thu, May 10, 2012 at 6:11 PM, Mark Shannon <mark at hotpy.org> wrote:
>> 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__).
> 
> It's the officially supported version of that API - the current
> version is solely a CPython implementation detail. The main change is
> moving exec_body to the end and making it optional, thus bringing the
> interface more in line with calling a metaclass directly. The name
> parameter is actually still there, I just forgot to include in the
> examples in the thread.
> 
> You'll find there's no mention of __build_class__ in the language or
> library references, thus there's currently no official way to
> programmatically define a new type in a way that complies with PEP
> 3115.
> 
> (This is explained in the tracker issue and the previous thread that
> proposed the name operator.build_class)

> 
> I prefer type.define(), but if the descriptor protocol does cause
> problems (and making static methods callable doesn't fix them), then
> we'll move it somewhere else (probably types.define() with a new
> _types module).

The problem with any non-overriding descriptor bound to type is that 
when accessed as type.define it acts as a descriptor, but when accessed
from any other class, say int.define it acts as a non-overriding 
meta-descriptor; c.f. type.mro() vs int.mro()

To avoid this problem, type.define needs to be an overriding descriptor
such as a property (a PyGetSetDef in C).
Alternatively, just make 'define' a non-descriptor.
It would unusual (unique?) to have a builtin-function (rather than a 
method-descriptor) bound to a class, but I can't see any fundamental 
reason not to.

Cheers,
Mark.