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

Decoration? Instantiation? Decorstantiation

>>> class Decorstantiator(object):
...    def __init__(self, func): self.func = func
...    def __call__(self): print "foo"; func()
...
>>> @Decorstantiator
... def bar(): print "bar"
...
>>> bar()
foo
bar

I can DoAnything

Sometimes you just want to test a piece of code that does deep calls into other objects in isolation.
For example:
def _change(self, op, val):
        self.parent.context.change(self.parent.model, self.attr, op, val)
How can we create an object to stand in for parent in the above code?
class DoAnything(object):
   def __call__(self, *a, **kw): return self
   __getattr__ = __add__ = __sub__ = __mul__ = __div__ = __call__
   #overload more operators to taste/requirements

>>> parent = DoAnything()
>>> parent.context.change(parent.model, "foo", "bar", "baz")
<__main__.DoAnything object at 0x02169B10>