[Python-3000] Compiling the PEP 3115 metaclass syntax

[Guido van Rossum]

The PEP proposes that the class statement accepts keyword arguments,
*args, and **kwds syntax as well as positional bases. This is a bit
messy to compile and execute, but we already have this, of course, in
the code for calling regular functions.

So I think it would be acceptable to this into a call to a new
(hidden) built-in function, named __build_class__. Then that this
class definition:

  class C(A, B, metaclass=M, other=42, *more_bases, *more_kwds):
    ...

would translate into this:

  C = __build_class__(<func>, 'C', A, B, metaclass=M, other=42,
*more_bases, *more_kwds)

where <func> is a function object for the class body. (It's a slightly
different function than currently; the current function *returns* the
locals, while the new one *takes* the locals as an argument; instead
of a LOAD_LOCALS opcode we need a STORE_LOCALS opcode.)

Then __build_class__ could be roughly like this (but implemented in C):

def __build_class__(func, name, *bases, metaclass=None, **kwds):
    if metaclass is None:
        metaclass = extract_metaclass(bases)  # may raise an exception
    prepare = getattr(metaclass, "__prepare__", None)
    if prepare:
        locals = prepare(name, bases, **kwds)
    else:
        locals = {}
    func(locals)
    return metaclass(name, bases, locals, **kwds)

What do folks think?

-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)