Singleton in Python Cookbook

Alex Popescu nospam.themindstorm at
Thu Jul 26 01:18:43 CEST 2007

"Diez B. Roggisch" <deets at> wrote in
news:5gq00tF3fm71gU1 at 

> Alex Popescu schrieb:
>> Hi all!
>> I was reading through Python Cookbook the Singleton recipe. At this
>> moment I am a bit puzzled as the example in the book is not working
>> resulting in: 
>> TypeError: type.__new__(SingleSpam): SingleSpam is not a subtype of
>> type 
>> (I haven't presented the original code as I am not sure about
>> copyrights). 
> AFAIK the cookbook is completely found online at ASPN. So no sweat 
> publishing it here.
> And regarding the problem: is it possible that you forgot to subclass 
> SingleSpam from object?
> Diez

The exact code:
class Singleton(object):
    """ A Pythonic Singleton """
    def _ _new_ _(cls, *args, **kwargs):
        if '_inst' not in vars(cls):
            cls._inst = type._ _new_ _(cls, *args, **kwargs)
        return cls._inst

if _ _name_ _ == '_ _main_ _':
    class SingleSpam(Singleton):
        def _ _init_ _(self, s): self.s = s
        def _ _str_ _(self): return self.s
    s1 = SingleSpam('spam')
    print id(s1), s1.spam( )
    s2 = SingleSpam('eggs')
    print id(s2), s2.spam( )

.w( the_mindstorm )p.

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