Optimize memory usage for common cases

Author: sobolevn

Lib/test/test_types.py

@@ -1506,7 +1506,14 @@ def __eq__(self, other):
                 other[key] = 1
                 return other
 
-        # exposes the internals of `MappingProxyType` via richcompare:
+        # Checks that it does not mutate the internals of `MappingProxyType`:
+        dc = {}
+        leaked = self.mappingproxy(dc) == Evil()
+        self.assertIs(type(leaked), dict)
+        self.assertIn(key, leaked)
+        self.assertNotIn(key, dc)
+
+        # Exposes the internals of `MappingProxyType` via richcompare:
         leaked = vars(list) == Evil()
         self.assertIs(type(leaked), dict)
         self.assertIn(key, leaked)

Objects/descrobject.c

@@ -1235,9 +1235,13 @@ mappingproxy_richcompare(PyObject *self, PyObject *w, int op)
     if (op == Py_EQ || op == Py_NE) {
         mappingproxyobject *v = (mappingproxyobject *)self;
         // We have to guard the mutable `dict` instances, because it can
-        // otherwise mutate the type's `__dict__` entries and cause crashes:
+        // otherwise mutate the type's `__dict__` entries and cause crashes.
+        // But, do not create copies on known types for memory optimization.
         // See gh-152405 on the details.
-        if (PyDict_CheckExact(v->mapping)) {
+        if (
+            PyDict_CheckExact(v->mapping) &&
+            !(PyAnyDict_CheckExact(w) || PyODict_CheckExact(w))
+        ) {
             // So, instead we send a copy:
             PyObject *copy = PyDict_Copy(v->mapping);
             if (copy == NULL) {