Singleton class: what is the purpose?

[Just]

In article <mailman.1054811769.993.python-list at python.org>,
 Gerrit Holl <gerrit at nl.linux.org> wrote:

> Guido's article on unifying types and classes mentions singleton classes.
> It says:
> 
> """
> As another example of __new__, here's a
> way to implement the singleton pattern.
> """
> (http://www.python.org/2.2.3/descrintro.html)
> 
> 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?

Yes.

> 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!")
>  20 ... # to do it correct, I think getrefcount would me neccesary
>  53 >>> Singleton()
> <__main__.Singleton object at 0x403afe4c>
>  54 >>> Singleton()
> Traceback (most recent call last):
>   File "<stdin>", line 1, in ?
>   File "<stdin>", line 7, in __init__
> Exception: No way jose!
>  55 >>> del Singleton.instances[0] # delete the last reference to the object
>  56 >>> Singleton()
> <__main__.Singleton object at 0x403affac>

It's not about enforcing it to be called only once, it's about 
guaranteeing you always get the same instance. Here's a Singleton object 
I recently wrote (it's simpler than Guido's -- I don't quite understand 
why he's mucking with __dict__ and adds an init() method):

  >>> class Singleton(object):
  ...     _instance = None
  ...     def __new__(cls, *args, **kwargs):
  ...         if cls._instance is None:
  ...             cls._instance = object.__new__(cls)
  ...         return cls._instance
  ... 
  >>> a = Singleton()
  >>> b = Singleton()
  >>> a is b
  1
  >>>

> But what is the purpose of a Singleton class?

In my case it was basically a euphemism for a global variable ;-)

Just