gh-77188: Name-mangled private methods don't roundtrip when pickling

Lib/test/picklecommon.py

@@ -351,3 +351,34 @@ def pie(self):
 class Subclass(tuple):
     class Nested(str):
         pass
+
+# For test_private_methods
+class PrivateMethods:
+    def __init__(self, value):
+        self.value = value
+
+    def __private_method(self):
+        return self.value
+
+    def get_method(self):
+        return self.__private_method
+
+    @classmethod
+    def get_unbound_method(cls):
+        return cls.__private_method
+
+    @classmethod
+    def __private_classmethod(cls):
+        return 43
+
+    @classmethod
+    def get_classmethod(cls):
+        return cls.__private_classmethod
+
+    @staticmethod
+    def __private_staticmethod():
+        return 44
+
+    @classmethod
+    def get_staticmethod(cls):
+        return cls.__private_staticmethod

Lib/test/pickletester.py

@@ -4117,6 +4117,21 @@ def test_c_methods(self):
                 with self.subTest(proto=proto, descr=descr):
                     self.assertRaises(TypeError, self.dumps, descr, proto)
 
+    def test_private_methods(self):
+        if self.py_version < (3, 15):
+            self.skipTest('not supported in Python < 3.15')
+        obj = PrivateMethods(42)
+        for proto in protocols:
+            with self.subTest(proto=proto):
+                unpickled = self.loads(self.dumps(obj.get_method(), proto))
+                self.assertEqual(unpickled(), 42)
+                unpickled = self.loads(self.dumps(obj.get_unbound_method(), proto))
+                self.assertEqual(unpickled(obj), 42)
+                unpickled = self.loads(self.dumps(obj.get_classmethod(), proto))
+                self.assertEqual(unpickled(), 43)
+                unpickled = self.loads(self.dumps(obj.get_staticmethod(), proto))
+                self.assertEqual(unpickled(), 44)
+
     def test_compat_pickle(self):
         if self.py_version < (3, 4):
             self.skipTest("doesn't work in Python < 3.4'")

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

@@ -7,6 +7,7 @@
 #include "pycore_object.h"
 #include "pycore_pyerrors.h"
 #include "pycore_pystate.h"       // _PyThreadState_GET()
+#include "pycore_symtable.h"      // _Py_Mangle()
 #include "pycore_weakref.h"       // FT_CLEAR_WEAKREFS()
 
 
@@ -140,9 +141,29 @@ method___reduce___impl(PyMethodObject *self)
     PyObject *funcself = PyMethod_GET_SELF(self);
     PyObject *func = PyMethod_GET_FUNCTION(self);
     PyObject *funcname = PyObject_GetAttr(func, &_Py_ID(__name__));
+    Py_ssize_t len;
     if (funcname == NULL) {
         return NULL;
     }
+    if (PyUnicode_Check(funcname) &&
+        (len = PyUnicode_GET_LENGTH(funcname)) > 2 &&
+        PyUnicode_READ_CHAR(funcname, 0) == '_' &&
+        PyUnicode_READ_CHAR(funcname, 1) == '_' &&
+        !(PyUnicode_READ_CHAR(funcname, len-1) == '_' &&
+          PyUnicode_READ_CHAR(funcname, len-2) == '_'))
+    {
+        PyObject *name = PyObject_GetAttr((PyObject *)Py_TYPE(funcself),
+                                          &_Py_ID(__name__));
+        if (name == NULL) {
+            Py_DECREF(funcname);
+            return NULL;
+        }
+        Py_SETREF(funcname, _Py_Mangle(name, funcname));
+        Py_DECREF(name);
+        if (funcname == NULL) {
+            return NULL;
+        }
+    }
     return Py_BuildValue(
             "N(ON)", _PyEval_GetBuiltin(&_Py_ID(getattr)), funcself, funcname);
 }