Singleton class: what is the purpose?

[Gordon Scott]

Is there any difference between the Borg and the Flyweight?  Sound like the
same thing.

Gordo

"Aldo Cortesi" <aldo at nullcube.com> wrote in message
news:mailman.1054814890.6876.python-list at python.org...
> Thus spake Gerrit Holl (gerrit at nl.linux.org):
>
> > What is a singleton class? What is the purpose of a
> > singleton class?
> >
> > On the web, I have found that a singleton class allows
> > only one instance.  Is that correct?
>
> Yep, that's right - the Singleton pattern is used to make
> sure that only one instance of a given class exists.
>
>
> > If so, it should be easily implementable with:
> >
> >  20 >>> class Singleton(object):
> >  20 ...  instances = []
> >  20 ...  def __init__(self):
> >  20 ...   if len(self.__class__.instances) == 0:
> >  20 ...    self.__class__.instances.append(self)
> >  20 ...   else:
> >  20 ...    raise Exception("No way jose!")
>
> Almost, but this is really only half an implementation of
> the Singleton pattern. A real implementation has to do two
> things:
>     - Ensure that only one instance exists
>     - Make that instance available in some global way
>
> See here for some discussion of various implementation
> strategies:
>
> http://aspn.activestate.com/ASPN/Python/Cookbook/Recipe/52558
>
>
> > But what is the purpose of a Singleton class?
>
> There are lots of circumstances in which the Singleton
> pattern comes in handy. For instance, you may have a program
> that should have only one connection to a database, or a
> system where only one print spooler should exist. Or,
> perhaps you want to share a single instance of a commonly
> used, but expensive to create, object between the various
> parts of your application. For a more in-depth discussion of
> the use (and abuse) of the Singleton Pattern see the
> excellent c2 wiki:
>
> http://c2.com/cgi/wiki?SingletonPattern
>
>
> You should also note that the Singleton pattern has recently
> fallen out of vogue with the Python in-crowd. If you're hip,
> and in-the-know, you're supposed to advocate the Borg
> pattern. The name is a pune or play on words - whereas the
> Singleton pattern means that There Can Only Be One (i.e.
> only one instance can exist), the Borg pattern means that
> You Will Be Assimilated (i.e. many instances can exist, but
> all have the same state). The skeleton for a Borg-ised class
> looks like this:
>
> class Borg:
>     __shared_state = {}
>     def __init__(self):
>         self.__dict__ = self.__shared_state
>
> As you can see, a single __dict__ is shared between all
> instances. In practice, this achieves much the same purpose
> as the Singleton pattern - object identity may differ from
> instance to instance, but since state is shared, all
> instances act exactly the same.
>
>
> Finally, many people forget that Python has something very
> much like the Singleton concept built in - modules. Python
> ensures that code in module scope is only executed once
> (unless you force a reload), no matter how many times a
> module is imported....
>
>
>
>
> Cheers,
>
>
>
> Aldo
>
>
>
>
> --
> Aldo Cortesi
> aldo at nullcube.com
> http://www.nullcube.com
>
>