Python singleton

singleton.py

class Singleton(object):
    """
    Singleton and parent for singletons
    """

    _instance = None
    _initialized = False

    def __new__(cls, *args, **kwargs):
        """
        Function called, when new instance of Identity is requested
        """
        if isinstance(cls._instance, cls):
            print("Instance of %s already exists" % cls.__name__)
            cls.__init__ = lambda *args, **kwargs: None
        else:
            print("Creating new instance of %s" % cls.__name__)
            cls._instance = object.__new__(cls, *args, **kwargs)
        
        return cls._instance


class InitializingSingleton(object):
    """
    Singleton and parent for singletons initializing instance everytime
    InitializingSingleton() is called. We will probably not need it.
    """

    _instance = None
    _initialized = False

    def __new__(cls, *args, **kwargs):
        """
        Function called, when new instance of Identity is requested
        """
        if isinstance(cls._instance, cls):
            print("Instance of %s already exists" % cls.__name__)
        else:
            print("Creating new instance of %s" % cls.__name__)
            cls._instance = object.__new__(cls, *args, **kwargs)
        
        return cls._instance


class Identity(Singleton):
    """
    Example of singleton (__init__ initialize only once)
    """
    def __init__(self, foo):
        print('Initializing %s' % self.__class__.__name__)
        self.foo = foo

    def __str__(self):
        return '%s, %s' % (id(self), self.foo)


class CertSorter(Singleton):
    """
    Another example of singleton (__init__ doesn't do anything)
    """
    def __init__(self):
        print('Initializing %s' % self.__class__.__name__)

    def __str__(self):
        return '%s' % id(self)

class ProfileManager(InitializingSingleton):
    """
    Last example of singleton (__init__ initialize everytime)
    """
    def __init__(self, foo):
        print('Initializing %s' % self.__class__.__name__)
        self.foo = foo

    def __str__(self):
        return '%s, %s' % (id(self), self.foo)

def main():
    """
    Example with singleton
    """
    ident_01 = Identity(foo="foo")
    print(ident_01)
    ident_02 = Identity(foo="bar")
    print(ident_02)
    ident_03 = Identity(foo="bar")
    print(ident_03)

    cs_01 = CertSorter()
    print(cs_01)
    cs_02 = CertSorter()
    print(cs_02)

    pm_01 = ProfileManager(foo="foo")
    print(pm_01)
    pm_02 = ProfileManager(foo="bar")
    print(pm_02)

if __name__ == '__main__':
    main()