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bpo-39778: Add clarification about tp_traverse and ownership (GH-18754) 

Automerge-Triggered-By: @pablogsal
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Pablo Galindo 2020-03-03 02:50:40 +00:00 committed by GitHub
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Doc/c-api/typeobj.rst
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@ -1192,7 +1192,7 @@ and :c:type:`PyType_Type` effectively act as defaults.)
The :c:member:`~PyTypeObject.tp_traverse` pointer is used by the garbage collector to detect The :c:member:`~PyTypeObject.tp_traverse` pointer is used by the garbage collector to detect
reference cycles. A typical implementation of a :c:member:`~PyTypeObject.tp_traverse` function reference cycles. A typical implementation of a :c:member:`~PyTypeObject.tp_traverse` function
simply calls :c:func:`Py_VISIT` on each of the instance's members that are Python simply calls :c:func:`Py_VISIT` on each of the instance's members that are Python
objects. For example, this is function :c:func:`local_traverse` from the objects that the instance owns. For example, this is function :c:func:`local_traverse` from the
:mod:`_thread` extension module:: :mod:`_thread` extension module::
static int static int
@ -1212,6 +1212,18 @@ and :c:type:`PyType_Type` effectively act as defaults.)
debugging aid you may want to visit it anyway just so the :mod:`gc` module's debugging aid you may want to visit it anyway just so the :mod:`gc` module's
:func:`~gc.get_referents` function will include it. :func:`~gc.get_referents` function will include it.
.. warning::
When implementing :c:member:`~PyTypeObject.tp_traverse`, only the members
that the instance *owns* (by having strong references to them) must be
visited. For instance, if an object supports weak references via the
:c:member:`~PyTypeObject.tp_weaklist` slot, the pointer supporting
the linked list (what *tp_weaklist* points to) must **not** be
visited as the instance does not directly own the weak references to itself
(the weakreference list is there to support the weak reference machinery,
but the instance has no strong reference to the elements inside it, as they
are allowed to be removed even if the instance is still alive).
Note that :c:func:`Py_VISIT` requires the *visit* and *arg* parameters to Note that :c:func:`Py_VISIT` requires the *visit* and *arg* parameters to
:c:func:`local_traverse` to have these specific names; don't name them just :c:func:`local_traverse` to have these specific names; don't name them just
anything. anything.