[Python-Dev] The Amazing Unreferenced Weakref (original) (raw)

Larry Hastings larry at hastings.org
Tue Sep 6 18:49:36 EDT 2016

This is all about current (3.6) trunk.

In Objects/weakrefobject.c, we have the function PyObject_ClearWeakRefs(). This is called when a generic object that supports weakrefs is destroyed; this is the code that calls the callbacks. Here's a little paragraph of code from the center:

for (i = 0; i < count; ++i) {
     PyWeakReference *next = current->wr_next;

     if (((PyObject *)current)->ob_refcnt > 0)
     {
         Py_INCREF(current);
         PyTuple_SET_ITEM(tuple, i * 2, (PyObject *) current);
         PyTuple_SET_ITEM(tuple, i * 2 + 1, current->wr_callback);
     }
     else {
         Py_DECREF(current->wr_callback);
     }
     current->wr_callback = NULL;
     clear_weakref(current);
     current = next;
}

"current" is the doubly-linked list of PyWeakReference objects stored inside the object that's getting destroyed.

My question: under what circumstances would ob_refcnt ever be 0? The tp_dealloc handler for PyWeakReference * objects removes it from this list and frees the memory. How could the reference count reach 0 without tp_dealloc being called and it being removed from the list?

Scratching my head like crazy,

//arry/

p.s. If you're thinking "why does he care?", understanding this would maybe help with the Gilectomy. So yes there's a point to this question. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.python.org/pipermail/python-dev/attachments/20160906/477518a9/attachment.html>

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