Puzzling behaviour of Py_IncRef
Tony Flury
tony.flury at btinternet.com
Thu Jan 27 02:46:11 EST 2022
On 26/01/2022 22:41, Barry wrote:
>
>
> Run python and your code under a debugger and check the ref count of
> the object as you step through the code.
>
> Don’t just step through your code but also step through the C python code.
> That will allow you to see how this works at a low level.
> Setting a watch point on the ref count will allow you run the code and
> just break as the ref count changes.
>
> That is what I do when a see odd c api behaviour.
>
> Barry
Thanks - I have tried a few times on a few projects to run a debugger in
mixed language mode and never had any success.
I will have to try again.
>> As posted in the original message - immediately before the call to
>> the C function/method sys.getrefcount reports the count to be 2
>> (meaning it is actually a 1).
>>
>> Inside the C function the ref count is incremented and the Py_REFCNT
>> macro reports the count as 3 inside the C function as expected (1 for
>> the name in the Python code, 1 for the argument as passed to the C
>> function, and 1 for the increment), so outside the function one would
>> expect the ref count to now be 2 (since the reference caused by
>> calling the function is then reversed).
>>
>> However - Immediately outside the C function and back in the Python
>> code sys.getrefcount reports the count to be 2 again - meaning it is
>> now really 1. So that means that the refcount has been decremented
>> twice in-between the return of the C function and the execution of
>> the immediate next python statement. I understand one of those
>> decrements - the parameter's ref count is incremented on the way in
>> so the same object is decremented on the way out (so that calls don't
>> leak references) but I don't understand where the second decrement is
>> coming from.
>>
>> Again there is nothing in the Python code that would cause that
>> decrement - the decrement behavior is in the Python runtime.
>>
>>>> --
>>>> Anthony Flury
>>>> email :anthony.flury at btinternet.com
>>>>
>>>> --
>>>> https://mail.python.org/mailman/listinfo/python-list
>>>>
>> --
>> Anthony Flury
>> email :anthony.flury at btinternet.com
--
Anthony Flury
email : anthony.flury at btinternet.com
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