issue with singleton python call two times __init__

Question

I have a singleton like this

class Singleton:

    class __impl:
        def __init__(self):
            print "INIT"

    __instance = None

    def __init__(self):
        # Check whether we already have an instance
        if Singleton.__instance is None:
            Singleton.__instance = Singleton.__impl()

        # Store instance reference as the only member in the handle
        self.__dict__['_Singleton__instance'] = Singleton.__instance

    def __getattr__(self, attr):
        """ Delegate access to implementation """
        return getattr(self.__instance, attr)

    def __setattr__(self, attr, value):
        """ Delegate access to implementation """
        return setattr(self.__instance, attr, value)

When I made several instances of Singleton I get two calls to init , I mean "INIT" is printed two times, and I supose that it shouldn't happened

Somebody has an idea of what is wrong with this or has a better way to implement this??

Answer 1

Here's a slightly simpler way to write a Singleton:

class Singleton(object):
    __instance = None
    def __new__(cls):
        if cls.__instance is None:
            cls.__instance = super(Singleton,cls).__new__(cls)
            cls.__instance.__initialized = False
        return cls.__instance

    def __init__(self):      
        if(self.__initialized): return
        self.__initialized = True
        print ("INIT")

a = Singleton()
b = Singleton()
print (a is b)

although there may be better ways. I have to admit that I've never been fond of singletons. I much prefer a factory type approach:

class Foo(object):
    pass

def foo_singleton_factory(_singlton = Foo()):
    return _singleton

a = foo_singleton_factory()
b = foo_singleton_factory()
print (a is b)

This has the advantage that you can keep getting the same instance of Foo if you want it, but you're not limited to a single instance if you decide 10 years down the road that you don't want a true singleton.

Answer 2

PEP 318 has an example of a singleton decorator for classes:

def singleton(cls):
    instances = {}
    def getinstance():
        if cls not in instances:
            instances[cls] = cls()
        return instances[cls]
    return getinstance

@singleton
class MyClass:
    ...

(though I haven't used it myself.)

Btw, about...

I made a singleton like this

Also, you should mention that you copied it directly from ActiveState.