Metaclasses broke in 2.2?

[Guido van Rossum]

[Drew Csillag]
> > > I don't know if this was intended, but it appears that the metaclass
> > > instantiation doesn't work with Python 2.2a1.  Are the old metaclass
> > > things no longer supported in 2.2 or is it just temporarily broken?

[Guido]
> > Can you be more specific?  What old metaclass are you referring to?
> > It's probably broken because I changed the Don Beaudry hook invocation
> > to favor the new metaclass mechanism that is central to the new types
> > in 2.2.

[Drew]
> Take any of the code in Demo/metaclasses in 2.2a1.  You get tracebacks
> of the variety:
> 
> Traceback (most recent call last):
>   File "Eiffel.py", line 113, in ?
>     _test()
>   File "Eiffel.py", line 101, in _test
>     class C(Eiffel):
> TypeError: cannot create 'instance' instances
> drew:~/build/Python-2.2a1/Demo/metaclasses>../../python Enum.py 
> Traceback (most recent call last):
>   File "Enum.py", line 169, in ?
>     _test()
>   File "Enum.py", line 128, in _test
>     class Color(Enum):
> TypeError: cannot create 'instance' instances
> drew:~/build/Python-2.2a1/Demo/metaclasses>
> 
> I guess the real question is: "how does one go about writing metaclasses
> in Python code in 2.2?"

I see.  You will be able to subclass from 'type' (or types.TypeType if
you prefer :-).  Right now this doesn't work right (neiter in 2.2a1
nor in CVS) so I can't post a working demo :-(

But here's something that you *should* be able to write:

class mytype(type):
    def __new__(*args):
        print "mytype.__new__" + str(args)
        return type.__new__(*args)

class C:
    __metaclass__ = mytype

print C.__class__		# <type 'mytype'>
a = C()
print a.__class__		# <type 'C'>

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