gistfile1.py

class Keeper(object):
    _instances = list()

    def __new__(cls):
        obj = object.__new__(cls)
        cls.__init__(obj)
        cls._instances.append(obj)

    @classmethod
    def list_instances(self):
        for instance in self._instances:
            yield instance

    def __str__(self):
        return '<%s instance at 0x%X>' % (self.__class__.__name__, id(self))