forevergarbage.py

>>> class A(object):
...  def __del__(self):
...   pass
...
>>> class B(object):
...  def __del__(self):
...   pass
...
>>> a = A()
>>> a.b = B()
>>> a.b.a = a
>>> import gc
>>> del a
>>> gc.collect()
4
>>> gc.garbage
[<__main__.A object at 0x1028e4710>, <__main__.B object at 0x1028e4750>]