Base class for singleton classes.

Any class derived from this one is a singleton. You can call its constructor multiple times, you'll get only one instance.

Note that `__init__` must not be defined. Use `init` instead. This is because `__init__` will always be called, thus reinitializing the state of your singleton each time you try to instanciate it. On the contrary, `init` will be called just one time.

To be sure that the `__init__` method won't be inadvertantly defined, the singleton will check for it at first instanciation and raise an `AssertionError` if it finds it.

Example:

        # This class is OK
        class A(Singleton):
            def init(self, val):
                self.val = val

        # This class will raise AssertionError at first instanciation, because
        # of the __init__ method.
        class B(Singleton):
            def __init__(self, val):
                self.val = val

:author: Laurent Burgbacher :contact: <lb@alawa.ch> :version: $Revision: 1.2 $

__new__ (
        cls,
        *args,
        *kwds,
        )

init (
        self,
        *args,
        *kwds,
        )