implementing a constant singleton

[robin and jim]

Is this the way to implement a constant singleton class using Python 2.2
"new-style" classes?

class Message(str):

   def __new__(this_class, specification):
      print 'Message.__new__ applied by %s' % str(this_class)
      return str.__new__(this_class, specification)

   def __init__(self, specification):
      print 'Message.__init__ applied by %s' % str(self)
      self.subject = specification[0]
      self.body = specification[1:]

   # additional Message class methods go here


class Constant_Singleton_Message(Message):

   def __new__(this_class):
      it = this_class.__dict__.get('__it__')
      if it is not None:
          return it
      this_class.__it__ = it = Message.__new__(this_class,
this_class.set_string())
      return it

##   def init(self):
##      Message.__init__(self, self.set_string())

   def __init__(self):
      Message.__init__(self, self.set_string())

   def set_string():
      return "I'm a constant singleton"

   set_string = staticmethod(set_string)

## end implementation--------------------------


I'm troubled by the fact that Message.__init__ is applied each time
Constant_Singleton_Message is "created", but I see no way to avoid it.

Thanks in advance for your feedback.