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[3.14] gh-123241: Document restrictions for tp_traverse implementations (gh-142272) (#142423)

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(cherry picked from commit 37988c57ea)

Co-authored-by: Sam Gross <colesbury@gmail.com>
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4
Doc/c-api/gcsupport.rst
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@ -232,6 +232,10 @@ The :c:member:`~PyTypeObject.tp_traverse` handler must have the following type:
object argument. If *visit* returns a non-zero value that value should be
returned immediately.
The traversal function must not have any side effects. Implementations
may not modify the reference counts of any Python objects nor create or
destroy any Python objects.
To simplify writing :c:member:`~PyTypeObject.tp_traverse` handlers, a :c:func:`Py_VISIT` macro is
provided. In order to use this macro, the :c:member:`~PyTypeObject.tp_traverse` implementation
must name its arguments exactly *visit* and *arg*:

5
Doc/c-api/typeobj.rst
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@ -1554,6 +1554,11 @@ and :c:data:`PyType_Type` effectively act as defaults.)
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).
.. warning::
The traversal function must not have any side effects. It must not
modify the reference counts of any Python objects nor create or destroy
any Python objects.
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
anything.