How does Mr. Martelli's Borg recipe work ?

[Mars]

I have looked long and hard at Mr. Martelli's Borg recipe:

http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/66531

It is a very useful substitute for a Singleton, but I can't figure out
how it works. _shared_state is never assigned any value, only
self.__dict__ is assigend self._shared_sate's value - or rather
(self.)_shared_state must be assigned a value at some point, otherwise
the code wouldn't work(right ?). I have gone through the code in the
debugger several(many) times, and inserted god knows how many print
statements, but without luck(or skill i guess).

Just so you don't waste your time, i do understand that _shared_state
is a class field(variable, or what not) and that its state is shared
by all instances.

Sorry if this question is banal, but I really need to understand this,
even if it means exposing my complete lack of understanding as far as
what is probaly basic Python knowledge.

Regards,

Martin