Eryk Sun added the comment:
For "del X.__new__", type_setattro in Objects/typeobject.c indirectly calls
update_one_slot. This finds object.__new__ fom the base object class when it
looks up __new__ on the type. Since __new__ for built-in types is special-cased
to be a built-in method instead of a slot wrapper, update_one_slot takes the
branch for "Py_TYPE(descr) == &PyCFunction_Type", which assigns the existing
tp_new to "specific". In this case that's slot_tp_new instead of object_new due
to the previous assignment of the "new" function to X.__new__.
slot_tp_new looks up and calls __new__, which in this case, as noted above, is
object.__new__. This built-in method calls tp_new_wrapper, which calls the
wrapped tp_new function. In this case that's object_new. If the type's tp_init
is object_init (i.e. not overridden) or tp_new is not object_new (i.e.
overridden), then object_new raises a TypeError when called with arguments. The
problem in this case is that __new__ isn't overridden anymore.
It seems to me that update_one_slot needs to install the tp_new that
tp_new_wrapper would call, e.g.
specific = (void *)(
(PyTypeObject *)PyCFunction_GET_SELF(descr))->tp_new;
In this case that's object_new. Thus after "del X.__new__", X would be restored
as if __new__ was never overridden.
----------
nosy: +eryksun
versions: +Python 2.7, Python 3.4, Python 3.6
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<http://bugs.python.org/issue25731>
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