Issue #14178: Problem deleting slices with steps != +1 in the _elementtree module.

Fixed the problem and added some tests. Closes #14178

Lib/test/test_xml_etree.py

 # xml.etree test.  This file contains enough tests to make sure that
 # all included components work as they should.
 # Large parts are extracted from the upstream test suite.
-
-# IMPORTANT: the same doctests are run from "test_xml_etree_c" in
-# order to ensure consistency between the C implementation and the
-# Python implementation.
+#
+# PLEASE write all new tests using the standard unittest infrastructure and
+# not doctest.
+#
+# IMPORTANT: the same tests are run from "test_xml_etree_c" in order
+# to ensure consistency between the C implementation and the Python
+# implementation.
 #
 # For this purpose, the module-level "ET" symbol is temporarily
 # monkey-patched when running the "test_xml_etree_c" test suite.
@@ -1948,6 +1951,81 @@ class NoAcceleratorTest(unittest.TestCase):
         self.assertEqual(pyET.Element.__module__, 'xml.etree.ElementTree')
         self.assertEqual(pyET.SubElement.__module__, 'xml.etree.ElementTree')
 
+
+class ElementSlicingTest(unittest.TestCase):
+    def _elem_tags(self, elemlist):
+        return [e.tag for e in elemlist]
+
+    def _subelem_tags(self, elem):
+        return self._elem_tags(list(elem))
+
+    def _make_elem_with_children(self, numchildren):
+        """Create an Element with a tag 'a', with the given amount of children
+           named 'a0', 'a1' ... and so on.
+
+        """
+        e = ET.Element('a')
+        for i in range(numchildren):
+            ET.SubElement(e, 'a%s' % i)
+        return e
+
+    def test_getslice_single_index(self):
+        e = self._make_elem_with_children(10)
+
+        self.assertEqual(e[1].tag, 'a1')
+        self.assertEqual(e[-2].tag, 'a8')
+
+        self.assertRaises(IndexError, lambda: e[12])
+
+    def test_getslice_range(self):
+        e = self._make_elem_with_children(6)
+
+        self.assertEqual(self._elem_tags(e[3:]), ['a3', 'a4', 'a5'])
+        self.assertEqual(self._elem_tags(e[3:6]), ['a3', 'a4', 'a5'])
+        self.assertEqual(self._elem_tags(e[3:16]), ['a3', 'a4', 'a5'])
+        self.assertEqual(self._elem_tags(e[3:5]), ['a3', 'a4'])
+        self.assertEqual(self._elem_tags(e[3:-1]), ['a3', 'a4'])
+        self.assertEqual(self._elem_tags(e[:2]), ['a0', 'a1'])
+
+    def test_getslice_steps(self):
+        e = self._make_elem_with_children(10)
+
+        self.assertEqual(self._elem_tags(e[8:10:1]), ['a8', 'a9'])
+        self.assertEqual(self._elem_tags(e[::3]), ['a0', 'a3', 'a6', 'a9'])
+        self.assertEqual(self._elem_tags(e[::8]), ['a0', 'a8'])
+        self.assertEqual(self._elem_tags(e[1::8]), ['a1', 'a9'])
+
+    def test_getslice_negative_steps(self):
+        e = self._make_elem_with_children(4)
+
+        self.assertEqual(self._elem_tags(e[::-1]), ['a3', 'a2', 'a1', 'a0'])
+        self.assertEqual(self._elem_tags(e[::-2]), ['a3', 'a1'])
+
+    def test_delslice(self):
+        e = self._make_elem_with_children(4)
+        del e[0:2]
+        self.assertEqual(self._subelem_tags(e), ['a2', 'a3'])
+
+        e = self._make_elem_with_children(4)
+        del e[0:]
+        self.assertEqual(self._subelem_tags(e), [])
+
+        e = self._make_elem_with_children(4)
+        del e[::-1]
+        self.assertEqual(self._subelem_tags(e), [])
+
+        e = self._make_elem_with_children(4)
+        del e[::-2]
+        self.assertEqual(self._subelem_tags(e), ['a0', 'a2'])
+
+        e = self._make_elem_with_children(4)
+        del e[1::2]
+        self.assertEqual(self._subelem_tags(e), ['a0', 'a2'])
+
+        e = self._make_elem_with_children(2)
+        del e[::2]
+        self.assertEqual(self._subelem_tags(e), ['a1'])
+
 # --------------------------------------------------------------------
 
 
@@ -1997,7 +2075,10 @@ def test_main(module=pyET):
     # The same doctests are used for both the Python and the C implementations
     test_xml_etree.ET = module
 
-    test_classes = [ElementTreeTest, TreeBuilderTest]
+    test_classes = [
+        ElementSlicingTest,
+        ElementTreeTest,
+        TreeBuilderTest]
     if module is pyET:
         # Run the tests specific to the Python implementation
         test_classes += [NoAcceleratorTest]

Modules/_elementtree.c

@@ -1343,9 +1343,74 @@ element_ass_subscr(PyObject* self_, PyObject* item, PyObject* value)
             return -1;
         }
 
-        if (value == NULL)
-            newlen = 0;
+        if (value == NULL) {
+            /* Delete slice */
+            size_t cur;
+            Py_ssize_t i;
+
+            if (slicelen <= 0)
+                return 0;
+
+            /* Since we're deleting, the direction of the range doesn't matter,
+             * so for simplicity make it always ascending.
+            */
+            if (step < 0) {
+                stop = start + 1;
+                start = stop + step * (slicelen - 1) - 1;
+                step = -step;
+            }
+
+            assert((size_t)slicelen <= PY_SIZE_MAX / sizeof(PyObject *));
+
+            /* recycle is a list that will contain all the children
+             * scheduled for removal.
+            */
+            if (!(recycle = PyList_New(slicelen))) {
+                PyErr_NoMemory();
+                return -1;
+            }
+
+            /* This loop walks over all the children that have to be deleted,
+             * with cur pointing at them. num_moved is the amount of children
+             * until the next deleted child that have to be "shifted down" to
+             * occupy the deleted's places.
+             * Note that in the ith iteration, shifting is done i+i places down
+             * because i children were already removed.
+            */
+            for (cur = start, i = 0; cur < (size_t)stop; cur += step, ++i) {
+                /* Compute how many children have to be moved, clipping at the
+                 * list end.
+                */
+                Py_ssize_t num_moved = step - 1;
+                if (cur + step >= (size_t)self->extra->length) {
+                    num_moved = self->extra->length - cur - 1;
+                }
+
+                PyList_SET_ITEM(recycle, i, self->extra->children[cur]);
+
+                memmove(
+                    self->extra->children + cur - i,
+                    self->extra->children + cur + 1,
+                    num_moved * sizeof(PyObject *));
+            }
+
+            /* Leftover "tail" after the last removed child */
+            cur = start + (size_t)slicelen * step;
+            if (cur < (size_t)self->extra->length) {
+                memmove(
+                    self->extra->children + cur - slicelen,
+                    self->extra->children + cur,
+                    (self->extra->length - cur) * sizeof(PyObject *));
+            }
+
+            self->extra->length -= slicelen;
+
+            /* Discard the recycle list with all the deleted sub-elements */
+            Py_XDECREF(recycle);
+            return 0;
+        }
         else {
+            /* A new slice is actually being assigned */
             seq = PySequence_Fast(value, "");
             if (!seq) {
                 PyErr_Format(
@@ -1367,7 +1432,6 @@ element_ass_subscr(PyObject* self_, PyObject* item, PyObject* value)
             return -1;
         }
 
-
         /* Resize before creating the recycle bin, to prevent refleaks. */
         if (newlen > slicelen) {
             if (element_resize(self, newlen - slicelen) < 0) {