gh-77188: Add support for pickling private methods and nested classes

Lib/test/pickletester.py

@@ -157,6 +157,15 @@ 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__"
@@ -2478,6 +2487,13 @@ 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

@@ -6,6 +6,7 @@
 #include "pycore_object.h"
 #include "pycore_pyerrors.h"
 #include "pycore_pystate.h"       // _PyThreadState_GET()
+#include "pycore_symtable.h"      // _Py_Mangle()
 
 
 #include "clinic/classobject.c.h"
@@ -134,9 +135,29 @@ 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);
 }