There is a dark, mysterious, and dangerous power of Necromancy in the Python language; used only at great peril: the __del__ method. Consider this class:
>>> class Living(object):
... def __del__(self):
... undead_hoard.append(self)
...
>>> undead_hoard = []
Let's use it.
>>> a = Living()
>>> undead_hoard
[]
So far so good. What happens if we delete one?
>>> del a
>>> undead_hoard
[<__main__.Living object at 0x020C01D0>]
The object got resurrected by __del__. No problem, it can be removed from the list:
>>> del undead_hoard[0]
>>> undead_hoard
[<__main__.Living object at 0x020C01D0>]
Hmm, maybe reset the list?
>>> undead_hoard = []
>>> undead_hoard
[<__main__.Living object at 0x020C01D0>]
Lesson one of Python Necromancy: __del__ may be called many times on the same object; a safe __del__ must be re-entrant. In this case, the resurrection was the fault of the __del__ method itself, but that is not always the case.
