[Python-Dev] PEP 318 - posting draft

[Alex Martelli]

On Wednesday 24 March 2004 09:01 pm, Skip Montanaro wrote:
>     >> class Foo(singleton):
>     >> ...
>     >> Foo = Foo()
>
>     Guido> Ok, so the metaclass would have to be a little different, but
>     Guido> this can be done with metaclasses.
>
> I'll take your word for it, but I wouldn't have the faintest idea how to do
> that.

Perhaps something like...:

class metaSingleton(type):
    def __new__(cls, classname, classbases, classdict):
        result = type.__new__(cls, classname, classbases, classdict)
        if classbases:
            return result()
        else:
            return result

class singleton:
    __metaclass__ = metaSingleton


class Foo(singleton):
    pass

print 'foo is', Foo, 'of type', type(Foo)


which outputs:

foo is <__main__.Foo object at 0x403f968c> of type <class '__main__.Foo'>

...?  (As to _why_ one would want such semantics, I dunno -- Singleton being 
most definitely _not_ my favourite DP, anyway...:-)


Alex