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A patch for PEP 3115 (metaclass syntax for Py3k). This enables full [arglist] syntax in the class header, e.g.
class C(B1, B2, metaclass=MC, *more_bases, **kwds):
 pass

Here's a new patch, which generates correct code for all variants of the class header syntax.
For now, I've implemented __build_class__ in Python; the built-in function by that name imports the build_class module and calls its build_class function. (I'll later redo this in C.)
This is still rough; I didn't even delete the old BUILD_CLASS opcode. But only a few unit tests fail (especially the ast and compiler tests). There are no unit tests for the new functionality. There is no documentation.
Failing tests:
 test_ast test_compiler test_ctypes test_descr test_grp test_pep292
File Added: meta.patch

New version. Fixes problems with bsddb, removes dead declaration from bltinmodule.c, and updates outdated docs for MAKE_FUNCTION and MAKE_CLOSURE in opcode.h while I was at it.
File Added: meta.patch

Here's a new version that implements __build_class__ in C. It's still an ordinary built-in function loaded using LOAD_GLOBAL so you can play games with this; however I'd rather keep that an implementation detail and not a feature (unlike overriding __import__, which *is* a feature).
One thing that's missing is support for __metaclass__, either in the module, or in the class. I don't want to support those (maybe the PEP should mention this), but we need a fixer for 2to3 to warn about this, at least (and to convert class C: __metaclass__ = M; ... into class C(metaclass=M): ...).
BTW, the test_grp failure mentioned earlier is bogus; that's an artefact of Google's LDAP. Some more tests probably fail, but I'll worry about those later.
File Added: meta.patch

New patch, gets rid of LOAD_LOCALS and BUILD_CLASS opcodes and adds LOAD_BUILD_CLASS opcode which avoids looking for __build_class__ in the globals; it only searches the builtins (like the IMPORT statement). Though this is debatable.
File Added: meta.patch

And another. This adds a unittest for the new build_class functionality, and fixes one bug (in the way __prepare__ was called) found this way. BTW doctest rocks for this purpose.
File Added: meta.patch

Fixed two bugs, with the help of (improved) test_metaclass.py:
- bug in symtable handling of keyword args that triggered an assert in debug mode
- bug in build_class that mutated the keyword args dict in place
I've got the feeling that there are leaks.
File Added: meta.patch

Fixed all library and test code I could find that used __metaclass__.
The only two remaining tests (without -uall) failing are test_ast and test_compiler.
File Added: meta.patch

Fix syntax error in pickletester.py
File Added: meta.patch

Checked in as revision 54428. The PEP is accepted.
(And yes, I realize this has been quite the monologue. :-)