> On 19 Jan 2022, at 10:57, Tony Flury via Python-list <python-list@python.org>
> wrote:
>
> I am writing a C extension module for an AVL tree, and I am trying to ensure
> reference counting is done correctly. I was having a problem with the
> reference counting so I worked up this little POC of the problem, and I hope
> someone can explain this.
The ref counting in the C API not easy to get right. Sometimes you must inc
some times you must not inc.
There are a lot of libraries that take that problem away from you.
For example I maintain PyCXX that is a C++ library that allows your drive the C
API without needed to worry about ref counting.
There are lots of other libraries as well that aim to do the same thing.
http://cxx.sourceforge.net/ <http://cxx.sourceforge.net/>
Barry
>
> 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);
> return self;
> }
>
> As you can see this function purely increments the reference count of the
> instance.
>
> /Note: I understand normally this would be the wrong this to do, but this is
> a POC of the issue, not live code. In the live code I am attaching a 2nd
> nodes to each other, and the live code therefore increments the ref-count for
> both objects - so even if the Python code deletes it's reference the
> reference count for the instance should still be 1 in order to ensure it
> doesn't get garbage collected./
>
> This function is exposed as the test_ref method.
>
> This is the test case :
>
> def test_000_009_test_ref_count(self):
> node = _Node("Hello")
> self.assertEqual(sys.getrefcount(node), 2)
> node.test_ref()
> self.assertEqual(sys.getrefcount(node), 3)
>
> The output of this test case is :
>
> test_000_009_test_ref_count (__main__.TestNode) ...
> Incrementing ref count for self - just for the hell of it
>
> before self has a ref count of 2
>
> after self has a ref count of 3
> FAIL
>
> ======================================================================
> FAIL: test_000_009_test_ref_count (__main__.TestNode)
> ----------------------------------------------------------------------
> Traceback (most recent call last):
> File "/home/tony/Development/python/orderedtree/tests/test_orderedtree.py",
> line 62, in test_000_009_test_ref_count
> self.assertEqual(sys.getrefcount(node), 3)
> AssertionError: 2 != 3
>
> So I understand why the first assert will be true - when the
> sys.getrefcount() function is called the ref count is incremented temporarily
> (as a borrowed reference), so there are now two references - the 'node'
> variable, and the borrowed reference in the function call.
>
> We then call the 'test_ref' method, and again that call causes a borrowed
> reference (hence the ref count being 2 initially within the method). The
> 'test_ref' method increments the reference of the instance - as you can see
> from the output we now have a ref count of 3 - (that count is the 'node'
> variable in the test case, the borrowed reference due to the method call, and
> the artificial increment from the 'ref_test' method).
>
> When the 'ref_test' method exits I would expect the ref count of the instance
> to now be 2 (one for the 'node' variable, and one as a result of the
> artificial increment increment').
>
> I would therefore expect the 2nd assertEqual in the test case to succeed. -
> in this case the borrowed reference within sys.getfrefcount() should cause
> the count to be 3.
>
> As you see though that 2nd assertEqual fails - suggesting that the refcount
> of 'node' is actually only 1 when the 'test_ref' method exits.
>
> Can someone explain why the 'test_ref' method fails to change the refcount of
> the 'node' instance.
>
> --
> https://mail.python.org/mailman/listinfo/python-list
--
https://mail.python.org/mailman/listinfo/python-list
