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Tue Dec 8 15:55:06 CET 2009 · https://mail.python.org/pipermail/python-dev/2009-December/094442.html

Kristján Valur Jónsson <kristjan <at> ccpgames.com> writes:
> 
> The problem with this is that once you have called
> factorial() once, you end up with a recursive cycle.

You don't need a closure to exhibit a reference cycle. A global function is 
enough:

>>> def helper(n):
...  if n:
...   return n*helper(n-1)
...  else:
...   return 1
... 
>>> helper.func_globals['helper'] is helper
True

If you really want to avoid this you can prevent the function from depending
on its outside environment:

>>> from functools import partial
>>> def helper2(rec, n):
...  if n:
...   return n*rec(rec, n-1)
...  else:
...   return 1
... 
>>> factorial = partial(helper2, helper2)
>>> "helper2" in factorial.func.func_globals
True
>>> del helper2
>>> "helper2" in factorial.func.func_globals
False
>>> factorial(3)
6

Regards

Antoine.

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[Python-Dev] recursive closures - reference cycle