Add Python 2 support for __metaclass__

[gvanrossum]

Now that we have metaclasses, we should support them for Python 2 as well. While in Python 3 the metaclass is specified as a metaclass=M keyword arg in the class heading, for Python 2 the metaclass is given by a __metaclass__ = M assignment in the body of the class.

[TRManderson]

Metaclasses in Python 2 are also often defined using:

class A(object):
    class __metaclass__(type):
        pass

[gvanrossum]

Ow, that's a terrible idiom... (And doesn't translate to Python 3.)

…

--Guido (mobile)

On Feb 7, 2017 4:25 PM, "Tom Manderson" ***@***.***> wrote: Metaclasses in Python 2 are also often defined using: class A(object): class __metaclass__(type): pass — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#2823 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ACwrMsDcusUllMkkinXdK8g6VnbGYRfIks5raQtzgaJpZM4L519l> .

[TRManderson]

Agreed, but it's valid and it works and people do it :'( :'( :'(

[TRManderson]

I think I've got a non-terrible solution for the __metaclass__ = M case, but the inner class thing going to require a structural change WRT metaclasses to support, so I'd say it's probably better to have an error message saying "don't do this" instead of trying to support it.

Just working on forming it into a nice PR at the moment.

[elazarg]

I think this issue can be closed by now