Why is this Python Borg / Singleton pattern working

Question

i just stumbled around the net and found these interesting code snipped:

http://code.activestate.com/recipes/66531/

class Borg:
    __shared_state = {}
    def __init__(self):
        self.__dict__ = self.__shared_state
    # and whatever else you want in your class -- that's all!

I understand what a singleton is but i don't understand that particular code snipped. Could you explain me how/where "__shared_state" is even changed at all?

I tried it in ipython:

In [1]: class Borg:
   ...:         __shared_state = {}
   ...:     def __init__(self):
   ...:             self.__dict__ = self.__shared_state
   ...:     # and whatever else you want in your class -- that's all!
   ...: 
In [2]: b1 = Borg()
In [3]: b2 = Borg()
In [4]: b1.foo="123"
In [5]: b2.foo
Out[5]: '123'
In [6]: 

but cannot fully understand how this could happen.

Answer 1

Because the class's instance's __dict__ is set equal to the __share_state dict. They point to the same object. (Classname.__dict__ holds all of the class attributes)

When you do:

b1.foo = "123"

You're modifying the dict that both b1.__dict__ and Borg.__shared_state refer to.

Answer 2

The __init__ method, which is called after instantiating any object, replaces the __dict__ attribute of the newly created object with the class attribute __shared_state.

a.__dict__, b.__dict__ and Borg._Borg__shared_state are all the same object. Note that we have to add the implicit prefix _Borg when accessing private attribute from outside the class.

In [89]: a.__dict__ is b.__dict__ is Borg._Borg__shared_state
Out[89]: True