Python singleton / object instantiation

Question

I'm learning Python and i've been trying to implement a Singleton-type class as a test. The code i have is as follows:

_Singleton__instance = None

class Singleton:
    def __init__(self):
        global __instance
        if __instance == None:           
            self.name = "The one"
            __instance = self
        else:
            self = __instance

This works in part but the self = __instance part seems to be failing. I've included some output from the interpretor to demonstrate (the code above is saved in singleton.py):

>>> import singleton
>>> x = singleton.Singleton()
>>> x.name
'The one'
>>> singleton._Singleton__instance.name
'The one'
>>> y = singleton.Singleton()
>>> y.name
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: Singleton instance has no attribute 'name'
>>> type(y)
<type 'instance'>
>>> dir(y)
['__doc__', '__init__', '__module__']

Is it possible to do what i'm trying? If not is there another way of doing this?

Any suggestions welcome.

Cheers.

Answer 1

Assigning to an argument or any other local variable (barename) cannot ever, possibly have ANY effect outside the function; that applies to your self = whatever as it would to ANY other assignment to a (barename) argument or other local variable.

Rather, override __new__:

class Singleton(object):

    __instance = None

    def __new__(cls):
        if cls.__instance == None:
            cls.__instance = object.__new__(cls)
            cls.__instance.name = "The one"
        return cls.__instance

I've done other enhancements here, such as uprooting the global, the old-style class, etc.

MUCH better is to use Borg (aka monostate) instead of your chosen Highlander (aka singleton), but that's a different issue from the one you're asking about;-).