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Date: Sat, 08 Sep 2012 22:22:55 +0100
From: MRAB <python@mrabarnett.plus.com>
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Subject: Re: set and dict iteration
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Xref: csiph.com comp.lang.python:28746

On 08/09/2012 21:06, Thomas Rachel wrote:
> Am 19.08.2012 00:14 schrieb MRAB:
>
>>> Can someone who is more familiar with the cycle detector and cycle
>>> breaker, help prove or disprove the above?
>>>
>> In simple terms, when you create an immutable object it can contain
>> only references to pre-existing objects, but in order to create a cycle
>> you need to make an object refer to another which is created later, so
>> it's not possible to create a cycle out of immutable objects.
>
> Yes, but if I add a list in-between, I can create a refcycle:
>
> a = []
> b = (a,)
> a.append(b)
>
> So b is a tuple consisting of one list which in turn contains b.
>
> It is not a direct cycle, but an indirect one.
>
> Or would that be detected via the list?
>
The quote was:

'''
...The tuple type does not implement a tp_clear function, because it’s 
possible to prove that no reference cycle can be composed entirely of 
tuples.
'''

Note: "composed entirely of tuples".

Or, in general, composed entirely of immutables.

Lists are not immutable, therefore the proof does not apply.