Decorator for making a class a singleton (without becoming a method)

loic_singleton.py

class Singleton(type):
    """
    Comme discuté sur la liste python@services.cnrs.fr (accès restreint à la communauté ESR)
     https://listes.services.cnrs.fr/wws/arc/python/2015-04/msg00006.html
    - Solution pour singleton co(?)-production Loïc Gouarin, Konrad Hinsen et al.
    """
    _instances = {}
    def __call__(self, *args, **kwargs):
        key = (str(self), args, str(kwargs))
        if key not in self._instances:
            self._instances[key] = super(Singleton, self).__call__(*args, **kwargs)
        self._instances
        return self._instances[key]

class A:
    """
    ma classe A
    """
    __metaclass__ = Singleton
    def __init__(self, x):
        self.x = x

class B(A):
    def __init__(self, x):
        super(B, self).__init__(x)

if __name__ == '__main__':
    a = A(1)
    b = A(1)
    c = A(2)
    d = B(1)

    print a is b
    print a is c
    print a is d

    import inspect
    print inspect.isclass(A)
    print inspect.isfunction(A)
    print inspect.isclass(B)