> On 25 Jan 2022, at 14:50, Tony Flury via Python-list <python-list@python.org> > wrote: > > >> On 20/01/2022 23:12, Chris Angelico wrote: >>> On Fri, 21 Jan 2022 at 10:10, Greg Ewing <greg.ew...@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.
The return self in C will not change the ref count. I would suggest setting a break point in your code and stepping out of the function and seeing that python’s code does to the ref count. Barry > > > > -- > Anthony Flury > email : anthony.fl...@btinternet.com > > -- > https://mail.python.org/mailman/listinfo/python-list > -- https://mail.python.org/mailman/listinfo/python-list
