multi-Singleton-like using __new__

[Arnaud Delobelle]

On Feb 9, 1:38 am, Freek Dijkstra <fr... at macfreek.nl> wrote:
> J Peyret wrote:
[...]
> I'll use __metaclass__ solution. Here is my (non-threaded) version:

Great! There's nothing voodoo about metaclasses.

> class RDFObject(object):
>     _cache = {}   # class variable is shared among all RDFObject
> instances
>     class __metaclass__(type):
>         def __call__(cls, *args, **kwargs):
>             return cls.__new__(cls, *args, **kwargs)
>     def __new__(cls, uri, *args, **kargs):
>         if uri not in cls._cache:
>             obj = object.__new__(cls)
>             cls._cache[uri] = obj
>             obj.__init__(uri, *args, **kargs)
>         return cls._cache[uri]
>     def __init__(self, uri):
>         self.uri = uri
>         print self.__class__, uri
>     # ...
[...]
> Perhaps indeed the try...except KeyError is even prettier (no idea
> about speed, but let's rename this thread if we want to discuss
> performance measurements).

I would go for something like (untested):

    def __new__(cls, uri, *args, **kwargs):
        obj = cls._cache.get(uri, None):
        if obj is None:
            obj = cls._cache[uri] = object.__new__(cls)
            obj.__init__(uri, *args, **kwargs)
        return obj

It doesn't look up the cache sho much and I think reads just as well.

--
Arnaud