From: Robin Narsingh Ranabhat Date: Fri, 22 Aug 2025 19:19:55 +0000 (+0545) Subject: gh-137740: Clarify `__del__` invocation mechanism in reference counting (#137741) X-Git-Tag: v3.15.0a1~608 X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=90b932e65080008dfd974b2e03c3068dbb72b95d;p=thirdparty%2FPython%2Fcpython.git gh-137740: Clarify `__del__` invocation mechanism in reference counting (#137741) Co-authored-by: Kumar Aditya --- diff --git a/Doc/extending/extending.rst b/Doc/extending/extending.rst index 17c6fb224265..f9b65643dfe8 100644 --- a/Doc/extending/extending.rst +++ b/Doc/extending/extending.rst @@ -1084,7 +1084,14 @@ references to all its items, so when item 1 is replaced, it has to dispose of the original item 1. Now let's suppose the original item 1 was an instance of a user-defined class, and let's further suppose that the class defined a :meth:`!__del__` method. If this class instance has a reference count of 1, -disposing of it will call its :meth:`!__del__` method. +disposing of it will call its :meth:`!__del__` method. Internally, +:c:func:`PyList_SetItem` calls :c:func:`Py_DECREF` on the replaced item, +which invokes replaced item's corresponding +:c:member:`~PyTypeObject.tp_dealloc` function. During +deallocation, :c:member:`~PyTypeObject.tp_dealloc` calls +:c:member:`~PyTypeObject.tp_finalize`, which is mapped to the +:meth:`!__del__` method for class instances (see :pep:`442`). This entire +sequence happens synchronously within the :c:func:`PyList_SetItem` call. Since it is written in Python, the :meth:`!__del__` method can execute arbitrary Python code. Could it perhaps do something to invalidate the reference to