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devel / comp.lang.python / Re: Puzzling behaviour of Py_IncRef

Re: Puzzling behaviour of Py_IncRef

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From: tony.flury@btinternet.com (Tony Flury)
Newsgroups: comp.lang.python
Subject: Re: Puzzling behaviour of Py_IncRef
Date: Wed, 19 Jan 2022 14:22:10 +0000
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by: Tony Flury - Wed, 19 Jan 2022 14:22 UTC

On 19/01/2022 11:09, Chris Angelico wrote:
> On Wed, Jan 19, 2022 at 10:00 PM Tony Flury via Python-list
> <python-list@python.org> wrote:
>> Extension function :
>>
>> static PyObject *_Node_test_ref_count(PyObject *self)
>> {
>> printf("\nIncrementing ref count for self - just for the hell
>> of it\n");
>> printf("\n before self has a ref count of %ld\n", Py_REFCNT(self));
>> Py_INCREF(self);
>> printf("\n after self has a ref count of %ld\n", Py_REFCNT(self));
>> fflush(stdout);
> At this point, the refcount has indeed been increased.
>
>> return self;
>> }
> And then you say "my return value is this object".
>
> The normal thing to do is to add a reference to whatever you're
> returning. For instance, Py_RETURN_NONE will incref None and then
> return it.
>
> So you're incrementing the refcount, then returning it without
> incrementing the refcount. Your code is actually equivalent to "return
> self".
>
> In order to actually leak a reference, you'd need to incref it twice.
>
> ChrisA

Chris - I am still puzzled - does doing 'return self' automatically
decrement the ref count of the object ?, and why is that the desired
behaviour ? Effectively it results in a decrement of two, since at the
exit of the function the ref count is only 1 (as witnessed by the
subsequent call to assertEqual).

(I am not suggesting that it should be changed - I understand that would
be a breaking change !).

You say I am returning it without incrementing, but I am explicitly
incrementing it before the return.

Subject	Author
Re: Puzzling behaviour of Py_IncRef	Tony Flury