Python-сообщество

__shared_state = {}
 def __init__(self):
 self.__dict__ = self.__shared_state
# that's all!

class Http():
    __single = {}
    url = ''

    def __init__(self):
        self.__dict__ = self.__single

    def load(self):
        print('http load url ' + self.url)

class Ac():

    def test(self):
        http = Http()
        http.load()

http = Http()
http.url = 'google.ru'

ac = Ac()
ac.test() # выводит http load url google.ru

>>> class test(object):
...     p={}
...     q=5
... 
>>> x=test()
>>> x.p[1]=2
>>> y=test()
>>> print x.p, x.q
{1: 2} 5
>>> print y.p, y.q
{1: 2} 5
>>> x.q=6
>>> print x.q, y.q
6 5
>>> print test.p
{1: 2}
>>> test.p[2]=4
>>> print test.p, x.p, y.p
{1: 2, 2: 4} {1: 2, 2: 4} {1: 2, 2: 4}
>>> test.p=0
>>> print test.p, x.p, y.p
0 0 0
>>> x.p=1
>>> print test.p, x.p, y.p
0 1 0
>>>

данные сохраняются за счет кортежей и списков являющихся переменными класса, а не экземпляра.

self.__dict__ = self.__single

class Singleton(type):
    ''' Singleton metaclass. Use by defining the metaclass of a class Singleton,
        e.g.: class ThereCanBeOnlyOne:
                  __metaclass__ = Singleton 
    '''              

    def __call__(mcs, *args, **kwargs):
        if not mcs.has_instance():
            mcs.instance = super(Singleton, mcs).__call__(*args, **kwargs)
        return mcs.instance

    def delete_instance(mcs):
        ''' Delete the (only) instance. This method is mainly for unittests so
            they can start with a clean slate. '''
        if mcs.has_instance():
            del mcs.instance

    def has_instance(mcs):
        ''' Has the (only) instance been created already? '''
        return hasattr(mcs, 'instance')