/* Python 3.10 source code */ static int list_ass_subscript(PyListObject* self, PyObject* item, PyObject* value) { if (_PyIndex_Check(item)) { /* ... */ } else if (PySlice_Check(item)) { Py_ssize_t start, stop, step, slicelength; if (PySlice_Unpack(item, &start, &stop, &step) < 0) { return -1; } slicelength = PySlice_AdjustIndices(Py_SIZE(self), &start, &stop, step); if (step == 1) return list_ass_slice(self, start, stop, value); /* Make sure s[5:2] = [..] inserts at the right place:  before 5, not before 2. */ if ((step < 0 && start < stop) || (step > 0 && start > stop)) stop = start; if (value == NULL) { /* ... */ } else { /* assign slice */ PyObject *ins, *seq; PyObject **garbage, **seqitems, **selfitems; Py_ssize_t i; size_t cur; /* protect against a[::-1] = a */ if (self == (PyListObject*)value) { seq = list_slice((PyListObject*)value, 0, PyList_GET_SIZE(value)); } else { seq = PySequence_Fast(value, // <-- call arbitrary code in python "must assign iterable " "to extended slice"); } if (!seq) return -1; /* ... */ selfitems = self->ob_item; seqitems = PySequence_Fast_ITEMS(seq); for (cur = start, i = 0; i < slicelength; cur += (size_t)step, i++) { garbage[i] = selfitems[cur]; ins = seqitems[i]; Py_INCREF(ins); selfitems[cur] = ins; // <-- maybe out of bounds } /* ... */ } /* ... */ }