gh-144995: Optimize memoryview == memoryview

Author: vstinner

Lib/test/test_memoryview.py

@@ -575,6 +575,27 @@ def test_array_assign(self):
         m[:] = new_a
         self.assertEqual(a, new_a)
 
+    def test_compare_equal(self):
+        # A memoryview is equal to itself: there is no need to compare
+        # individual values. This is not true for float values since they can
+        # be NaN, and NaN is not equal to itself.
+        for int_format in 'bBhHiIlLqQ':
+            with self.subTest(format=int_format):
+                a = array.array(int_format, [1, 2, 3])
+                m = memoryview(a)
+                self.assertTrue(m == m)
+
+        for float_format in 'fd':
+            with self.subTest(format=int_format):
+                a = array.array(float_format, [1.0, 2.0, float('nan')])
+                m = memoryview(a)
+                # nan is not equal to nan
+                self.assertFalse(m == m)
+
+                a = array.array(float_format, [1.0, 2.0, 3.0])
+                m = memoryview(a)
+                self.assertTrue(m == m)
+
 
 class BytesMemorySliceTest(unittest.TestCase,
     BaseMemorySliceTests, BaseBytesMemoryTests):

Misc/NEWS.d/next/Core_and_Builtins/2026-02-19-12-49-15.gh-issue-144995.Ob2oYJ.rst

@@ -0,0 +1,2 @@
+Optimize :class:`memoryview` comparison: a :class:`memoryview` is equal to
+itself, there is no need to compare values. Patch by Victor Stinner.

Objects/memoryobject.c

@@ -3101,6 +3101,25 @@ cmp_rec(const char *p, const char *q,
     return 1;
 }
 
+static int
+is_float_format(const char *format)
+{
+    if (format == NULL) {
+        return 0;
+    }
+    if (strcmp("d", format) == 0) {
+        return 1;
+    }
+    if (strcmp("f", format) == 0) {
+        return 1;
+    }
+    if (strcmp("e", format) == 0) {
+        return 1;
+    }
+    return 0;
+}
+
+
 static PyObject *
 memory_richcompare(PyObject *v, PyObject *w, int op)
 {
@@ -3122,6 +3141,14 @@ memory_richcompare(PyObject *v, PyObject *w, int op)
     }
     vv = VIEW_ADDR(v);
 
+    // A memoryview is equal to itself: there is no need to compare individual
+    // values. This is not true for float values since they can be NaN, and NaN
+    // is not equal to itself.
+    if (v == w && !is_float_format(vv->format)) {
+        equal = 1;
+        goto result;
+    }
+
     if (PyMemoryView_Check(w)) {
         if (BASE_INACCESSIBLE(w)) {
             equal = (v == w);