bpo-33007: Name-mangled private methods don't roundtrip when pickling

Author: ZackerySpytz

Lib/test/pickletester.py

@@ -150,6 +150,15 @@ def __reduce__(self):
         # Shouldn't support the recursion itself
         return K, (self.value,)
 
+
+class L:
+    def __init__(self):
+        self.attr = self.__foo
+
+    def __foo(self):
+        return 42
+
+
 import __main__
 __main__.C = C
 C.__module__ = "__main__"
@@ -2249,6 +2258,13 @@ def test_many_puts_and_gets(self):
                 loaded = self.loads(dumped)
                 self.assert_is_copy(obj, loaded)
 
+    def test_mangled_private_methods_roundtrip(self):
+        obj = L()
+        for proto in protocols:
+            with self.subTest(proto=proto):
+                loaded = self.loads(self.dumps(obj, proto))
+                self.assertEqual(loaded.attr(), 42)
+
     def test_attribute_name_interning(self):
         # Test that attribute names of pickled objects are interned when
         # unpickling.

Misc/NEWS.d/next/Library/2020-07-14-23-54-18.bpo-33007.TyI3_Q.rst

@@ -0,0 +1 @@
+The :mod:`pickle` module now properly handles name-mangled private methods.

Objects/classobject.c

@@ -128,8 +128,20 @@ method_reduce(PyMethodObject *im, PyObject *Py_UNUSED(ignored))
     if (funcname == NULL) {
         return NULL;
     }
+    PyObject *name = _PyObject_GetAttrId((PyObject *)Py_TYPE(self),
+                                         &PyId___name__);
+    if (name == NULL) {
+        Py_DECREF(funcname);
+        return NULL;
+    }
+    PyObject *mangled = _Py_Mangle(name, funcname);
+    Py_DECREF(name);
+    Py_DECREF(funcname);
+    if (mangled == NULL) {
+        return NULL;
+    }
     return Py_BuildValue("N(ON)", _PyEval_GetBuiltinId(&PyId_getattr),
-                         self, funcname);
+                         self, mangled);
 }
 
 static PyMethodDef method_methods[] = {