Puzzling behaviour of Py_IncRef

[Tony Flury]

On 20/01/2022 23:12, Chris Angelico wrote:
> On Fri, 21 Jan 2022 at 10:10, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:
>> On 20/01/22 12:09 am, Chris Angelico wrote:
>>> At this point, the refcount has indeed been increased.
>>>
>>>>            return self;
>>>>       }
>>> And then you say "my return value is this object".
>>>
>>> So you're incrementing the refcount, then returning it without
>>> incrementing the refcount. Your code is actually equivalent to "return
>>> self".
>> Chris, you're not making any sense. This is C code, so there's no
>> way that "return x" can change the reference count of x.
> Yeah, I wasn't clear there. It was equivalent to *the Python code*
> "return self". My apologies.
>
>>   > 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.
>>   >
>>
>> The OP understands that this is not a normal thing to do. He's
>> trying to deliberately leak a reference for the purpose of diagnosing
>> a problem.
>>
>> It would be interesting to see what the actual refcount is after
>> calling this function.

After calling this without a double increment in the function the ref 
count is still only 1 - which means that the 'return self' effectively 
does a double decrement. My original message includes the Python code 
which calls this 'leaky' function and you can see that despite the 
'leaky POC' doing an increment ref count drops back to one after the return.

You are right this is not a normal thing to do, I am trying to 
understand the behaviour so my library does the correct thing in all 
cases - for example - imagine you have two nodes in a tree :

A --- > B

And your Python code has a named reference to A, and B also maintains a 
reference to A as it's parent.

In this case I would expect A to have a reference count of 2 (counted as 
3 through sys.getrefcount() - one for the named reference in the Python 
code - and one for the link from B back to A; I would also expect B to 
have a reference count here of 1 (just the reference from A - assuming 
nothing else referenced B).

My original code was incrementing the ref counts of A and B and then 
returning A. within the Python test code A had a refcount of 1 (and not 
the expected 2), but the refcount from B was correct as far as I could tell.


> Yes, and that's why I was saying it would need a *second* incref.
>
> ChrisA

Thank you to all of you for trying to help - I accept that the only way 
to make the code work is to do a 2nd increment.

I don't understand why doing a 'return self' would result in a double 
decrement - that seems utterly bizzare behaviour - it obviously works, 
but why.



-- 
Anthony Flury
email : anthony.flury at btinternet.com