bpo-40889: Optimize dict.items() ^ dict.items() (GH-20718)

Lib/test/test_dict.py

@@ -697,6 +697,16 @@ class DictTest(unittest.TestCase):
         self.assertEqual(k1 ^ k2, {(3,3)})
         self.assertEqual(k1 ^ k3, {(1,1), (2,2), (4,4)})
 
+    def test_items_symmetric_difference(self):
+        rr = random.randrange
+        for _ in range(100):
+            left = {x:rr(3) for x in range(20) if rr(2)}
+            right = {x:rr(3) for x in range(20) if rr(2)}
+            with self.subTest(left=left, right=right):
+                expected = set(left.items()) ^ set(right.items())
+                actual = left.items() ^ right.items()
+                self.assertEqual(actual, expected)
+
     def test_dictview_mixed_set_operations(self):
         # Just a few for .keys()
         self.assertTrue({1:1}.keys() == {1})

Misc/NEWS.d/next/Core and Builtins/2020-06-08-22-46-33.bpo-40889.vIBl-W.rst

@@ -0,0 +1 @@
+Improved the performance of symmetric difference operations on dictionary item views.  Patch by Dennis Sweeney.

Objects/dictobject.c

@@ -4409,9 +4409,99 @@ dictviews_or(PyObject* self, PyObject *other)
     return result;
 }
 
+static PyObject *
+dictitems_xor(PyObject *self, PyObject *other)
+{
+    assert(PyDictItems_Check(self));
+    assert(PyDictItems_Check(other));
+    PyObject *d1 = (PyObject *)((_PyDictViewObject *)self)->dv_dict;
+    PyObject *d2 = (PyObject *)((_PyDictViewObject *)other)->dv_dict;
+
+    PyObject *temp_dict = PyDict_Copy(d1);
+    if (temp_dict == NULL) {
+        return NULL;
+    }
+    PyObject *result_set = PySet_New(NULL);
+    if (result_set == NULL) {
+        Py_CLEAR(temp_dict);
+        return NULL;
+    }
+
+    PyObject *key = NULL, *val1 = NULL, *val2 = NULL;
+    Py_ssize_t pos = 0;
+    Py_hash_t hash;
+
+    while (_PyDict_Next(d2, &pos, &key, &val2, &hash)) {
+        Py_INCREF(key);
+        Py_INCREF(val2);
+        val1 = _PyDict_GetItem_KnownHash(temp_dict, key, hash);
+
+        int to_delete;
+        if (val1 == NULL) {
+            if (PyErr_Occurred()) {
+                goto error;
+            }
+            to_delete = 0;
+        }
+        else {
+            Py_INCREF(val1);
+            to_delete = PyObject_RichCompareBool(val1, val2, Py_EQ);
+            if (to_delete < 0) {
+                goto error;
+            }
+        }
+
+        if (to_delete) {
+            if (_PyDict_DelItem_KnownHash(temp_dict, key, hash) < 0) {
+                goto error;
+            }
+        }
+        else {
+            PyObject *pair = PyTuple_Pack(2, key, val2);
+            if (pair == NULL) {
+                goto error;
+            }
+            if (PySet_Add(result_set, pair) < 0) {
+                Py_DECREF(pair);
+                goto error;
+            }
+            Py_DECREF(pair);
+        }
+        Py_DECREF(key);
+        Py_XDECREF(val1);
+        Py_DECREF(val2);
+    }
+    key = val1 = val2 = NULL;
+
+    _Py_IDENTIFIER(items);
+    PyObject *remaining_pairs = _PyObject_CallMethodIdNoArgs(temp_dict,
+                                                             &PyId_items);
+    if (remaining_pairs == NULL) {
+        goto error;
+    }
+    if (_PySet_Update(result_set, remaining_pairs) < 0) {
+        Py_DECREF(remaining_pairs);
+        goto error;
+    }
+    Py_DECREF(temp_dict);
+    Py_DECREF(remaining_pairs);
+    return result_set;
+
+error:
+    Py_XDECREF(temp_dict);
+    Py_XDECREF(result_set);
+    Py_XDECREF(key);
+    Py_XDECREF(val1);
+    Py_XDECREF(val2);
+    return NULL;
+}
+
 static PyObject*
 dictviews_xor(PyObject* self, PyObject *other)
 {
+    if (PyDictItems_Check(self) && PyDictItems_Check(other)) {
+        return dictitems_xor(self, other);
+    }
     PyObject *result = dictviews_to_set(self);
     if (result == NULL) {
         return NULL;