Class is Set of Instances

This shows how to make a class also be a set of its own instances.
A conceptually simpler way of maintaining a global registry of instances.

>>> class SetMetaClass(type):
...    def __new__(cls, *a, **kw):
...       cls.all = set()
...       return type.__new__(cls, *a, **kw)
...    def __iter__(cls): return cls.all.__iter__()
...    def __len__(cls): return len(cls.all)
...    def __contains__(cls, e): return e in cls.all
...    def add(cls, e): cls.all.add(e)
...    def discard(cls, e): cls.all.discard(e)
...
>>> class InstanceSet(object):
...    __metaclass__ = SetMetaClass
...    def __init__(self, *a, **kw):
...       self.__class__.add(self)
...
>>> InstanceSet()
<__main__.InstanceSet object at 0x00DAB0F0>
>>> InstanceSet()
<__main__.InstanceSet object at 0x00D23610>
>>> [a for a in InstanceSet]
[<__main__.InstanceSet object at 0x00DAB0F0>, <__main__.InstanceSet object at 0x
00D23610>]
>>> len(InstanceSet)
2