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Haven't looked at the patches, but your comment "The patch
also addresses PySequence_InPlaceConcat/Repeat()
but without testing them -- they can't be tested from
Python, so they are already completely untested." is not
quite correct:

We have the _testcapimodule.c for doing C API tests.

Do you also address the problem I posted to python-dev ?

"""...the following code
could be the cause for leaking a NotImplemented singleton
reference:

#define SLOT1BINFULL(FUNCNAME, TESTFUNC, SLOTNAME, OPSTR,
ROPSTR) \
static PyObject * \
FUNCNAME(PyObject *self, PyObject *other) \
{ \
	static PyObject *cache_str, *rcache_str; \
	int do_other = self->ob_type != other->ob_type && \
	    other->ob_type->tp_as_number != NULL && \
	    other->ob_type->tp_as_number->SLOTNAME == TESTFUNC; \
	if (self->ob_type->tp_as_number != NULL && \
	    self->ob_type->tp_as_number->SLOTNAME == TESTFUNC) { \
		PyObject *r; \
		if (do_other && \
		    PyType_IsSubtype(other->ob_type, self->ob_type) && \
		    method_is_overloaded(self, other, ROPSTR)) { \
			r = call_maybe( \
				other, ROPSTR, &rcache_str, "(O)", self); \
			if (r != Py_NotImplemented) \
				return r; \
			Py_DECREF(r); \
			do_other = 0; \
		} \
		r = call_maybe( \
			self, OPSTR, &cache_str, "(O)", other); \
		if (r != Py_NotImplemented || \
		    other->ob_type == self->ob_type) \
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
If both types are of the same type, then a NotImplemented
returng
value would be returned.

			return r; \
		Py_DECREF(r); \
	} \
	if (do_other) { \
		return call_maybe( \
			other, ROPSTR, &rcache_str, "(O)", self); \
	} \
	Py_INCREF(Py_NotImplemented); \
	return Py_NotImplemented; \
}

"""